"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)