In Part 2 of "Elle a banni le sucre pendant un an," Danièle Gerkens, a journalist at Elle magazine, talks about the health benefits she experienced after cutting sugar from her diet for one year. When the year was almost up, she was expecting to break her sugar fast with mountains of whipped cream, but it was actually a single piece of dark chocolate that did her in:

BANNER PLACEHOLDER

Je me disais que j'allais me rouler dans la chantilly, et cetera.

I told myself that I was going to wallow in whipped cream, et cetera.

Et puis en fait, plus ça arrivait,

And then in fact, the closer it came [to the end],

plus je me disais, mais... qu'est-ce que je vais faire?

the more I was wondering, but... what am I going to do?

Captions 102-104, Le Figaro - Elle a banni le sucre pendant un an

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Note the two different translations of je me disais here: "I told myself" and "I was wondering." The reflexive verb se dire can mean a number of things depending on context, namely "to tell/say to oneself" and "to wonder/think." In a sense, these both mean the same thing: when you wonder or think about something, you're telling yourself about it. 

When multiple people se disent, they could be thinking about something or telling themselves something, but they could also just be talking to each other: 

Christine et Alice sont de meilleures amies. Elles se disent tout. 
Christine and Alice are best friends. They tell each other everything.

Se dire can also mean "to say of oneself," or in other words, "to claim to be":

Le Charles de Gaulle,

Charles de Gaulle,

où la direction se dit d'abord victime de son image.

where the management claims first to be a victim of its image.

Caption 29, Le Journal - Hôpital ultra-moderne à Burkina Faso

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Or se dire can simply mean "to be said," which has a few different connotations. Here Danièle is (somewhat cheekily) talking about something she thinks is taboo and can't be mentioned in public. Believe it or not, she's referring to her love of milk chocolate! 

Je sais, ça se dit pas, mais j'adorais ça.

I know you're not supposed to say it, but that's what I loved.

Caption 112, Le Figaro - Elle a banni le sucre pendant un an

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In its most general sense, se dire refers to anything that "is said" in everyday language:

Par contre,

On the other hand,

"faire le beau" se dit d'un chien qui se tient sur les pattes arrière

"faire le beau" is said of a dog that stands on its hind legs

pour réclamer un sucre.

to beg for a lump of sugar.

Captions 24-25, Margaux et Manon - Emplois du verbe faire

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"Je n'ai pas des biscuits": ça se dit en français? -Non. Il faut dire: "je n'ai pas de biscuits".
Can you say je n'ai pas des biscuits in French? -No. You have to say je n'ai pas de biscuits [I don't have any cookies].

BANNER PLACEHOLDER

Don't confuse ça se dit with ça te dit (or ça vous dit in the plural), which means "how does that sound" or "how would you like..." (literally, "does it speak to you"):

Ça te dit de réviser les multiples sens de l'expression "se dire"? 
How would you like to review the multiple meanings of the expression se dire?