"Retain" is a formal word which means "keep". Here are some examples of sentences in which you can use "retain":

Many nursing mothers retain much of the weight that they gained during pregnancy.

Tests like these just measure your ability to acquire and retain knowledge. They're not a true test of a person's skill or expertise.

Here are some of the most common things that are "retained':

(someone) retains power

(a company) retains employees

(something) retains heat

(something) retains moisture

(someone) retains ownership of (something)