The expression au niveau de means "at the level of" or "on the level of." You can use this expression to talk about something that's physically level with something else:

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...pour avoir de l'eau au niveau des genoux,

...having the water at knee level,

vous allez être emporté de ce côté.

you are going to be carried away to this side.

Captions 12-13, À la plage avec Lionel - La plage

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La ville est au niveau de la mer.

The city is at sea level.

Or, as in English, it can refer to more general things, such as one's health or one's skills or abilities:

Ben, c'est vrai qu'au niveau de la santé,

Well, it's true that on a health level,

 je le ressens parfois.

I feel it sometimes.

Captions 80-81, Amal et Caroline - La cigarette

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Je ne suis pas au niveau des autres élèves.

I'm not at the (same) level as the other students.

Another way of saying "on a health level" is au niveau sanitaire. You'll often see "au niveau + adjective" (no de) used in this way: au niveau national (on a national level), au niveau économique (on an economic level), au niveau spirituel (on a spiritual level), etc.

But sometimes "on the level of" or "on an x level" isn't the most succinct translation of au niveau de. It's also equivalent to phrases such as "when it comes to," "regarding," and "in regards to":

Parce que... en France

Because... in France

on a souvent tendance

we often have a tendency

à faire des amalgames

to mix them together

en particulier au niveau du sandwich, du kebab...

particularly when it comes to sandwiches, kebabs...

-Au niveau des fromages...

-When it comes to cheeses...

Captions 54-57, Lionel et J.B. - La salade grecque

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Ensuite au niveau de la selle,

Then regarding the seat,

faut bien la régler à votre hauteur.

you should really adjust it to your height.

Captions 35-36, Amal - Vélib

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Even when referring to physical spaces, au niveau de doesn't necessarily imply that something is level with something else. It could just mean "near," "by," or "in the region/area of":

Bruce se rend compte qu'un autre cours d'eau rejoint son

Bruce realized that another river joined his

Nil au niveau de Khartoum.

Nile near Khartoum.

Caption 42, Il était une fois - les Explorateurs - 15. Bruce et les sources du Nil

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Au niveau de also functions as a simple preposition when used with body parts, in which case it means "in":

Je ressens une douleur au niveau de mon genou.

I feel a pain in my knee.

No matter your niveau de français, au niveau de is a great expression to know! 

Animals are generally (and perhaps unjustly) considered to be less intelligent than humans, which explains why the French word bête can mean both "beast" and "stupid":

Après tout, c'est bête la guerre.

After all, war is stupid.

Caption 25, Il était une fois: les Explorateurs - 15. Bruce et les sources du Nil

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The related noun bêtise can mean anything along the lines of "stupidity" or "idiocy." You can use it in a general sense to talk about "something stupid":

Après les parents, ils me disent, quand ils font une bêtise...

Later the parents tell me, when they do something stupid...

Caption 56, Banlieues françaises - jeunes et policiers, l'impossible réconciliation?

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Or you might use it to refer to something more specific, such as a mistake. Une bêtise isn't just any old mistake, but a particularly stupid one:

Vous allez réparer vos bêtises.

You're going to repair your stupid mistakes.

Caption 31, Il était une fois: Notre Terre - 9. Les écosystèmes

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Of course, if you tell someone he or she has made a stupid mistake, you could be implying that the person him or herself is stupid. Une erreur is a more neutral word for "mistake" that doesn't connote stupidity:

Elle fait une terrible erreur.

She's making a terrible mistake.

Caption 4, Le Jour où tout a basculé - Mes grands-parents sont infidèles

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The plural bêtises is often used to refer to "nonsense," "mischief," or any kind of naughty behavior:

Arrête tes bêtises.

Stop your nonsense.

Mais si on fait des bêtises, on sait jamais...

But if we get into mischief, you never know...

Caption 90, Actu Vingtième - Le Repas des anciens

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If you argue with someone over des bêtises, you're arguing over nothing:

Mes enfants se disputent toujours pour des bêtises.
My kids are always arguing with each other over nothing.

When it comes to learning a language, there's no such thing as a stupid mistake. So don't fret if you forget an accent mark or type in the wrong word in a Yabla game—you've just made a simple erreur, not une bêtise!

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For fun, here's an 80s throwback for you: Sabine Paturel's "Les Bêtises," which was a smash hit in France in 1986.