The most common character in Chinese is 的 (de). That’s because 的 is used all the time to mark possession. That means that it’s used to talk about things belonging to other things, or to attach qualities to things.

的 is extremely versatile in Chinese. Pretty much any relationship where one thing belongs to another, or is the property of another, can be described with 的.

的 is kind of equivalent to ’s (apostrophe s) in English. It goes between two things to indicate possession. But it’s used very widely to attach any kind of quality or possession.

Some examples:

Zhè shì nǐde.
This is yours.
Nà shì Xiǎo Lǐ de shū.
That is Xiao Li's book.
Zhè shì wǒde diànhuà hàomǎ.
This is my phone number.
Zhè shì tāmende fángzi.
This is their house.
Zhè tiáo kùzi shì hēisè de.
These trousers are black.
Tā shì yīgè hěn zhòngyào de rén.
She is a very important person.
You might want to have a look at a more challenging example sentence for 的. Don’t worry if this is beyond your ability at the moment, though:

Wǒ jīntiān xué de dōngxi hěn yǒuyìsi.
The things I have learnt today are very interesting.
The main point to remember is that 的 can attach pretty much anything to anything else. You’ll get more used to it the more you read and listen to Chinese.