Example 1:

My boyfriend loves action movies, but I prefer romantic comedies. He gets so excited when we go to the theater to see the newest action movie. I don’t want to make him feel bad, so I fake it and act excited, too.

Example 2:

Bonnie: Shhh! Look, little Joey is sleeping. Let’s be very quiet. I don’t want to wake him up. 

Amanda: OK, I’ll be quiet.

Joey: HAHA! I tricked you! I’m not sleeping!

Bonnie: Joey! You were faking it! You tricked us!

Meaning:

Remember the word fake (adj)means “not real.”
The idiom to fake it means to pretend, and is used as a verb. 

In example 1, the speaker’s excitement for action movies is fake, she doesn’t really enjoy them. When she goes to see an action movie with her boyfriend, she fakes it/pretends

In example 2, Joey is pretending to sleep, meaning that his sleeping is fake. He is faking it and not actually sleeping. He just wanted to play a joke on Bonnie and Amanda. 

This idiom comes from the LSI book titled Reading Transitions.  This book is used to teach Level 4 Reading/Vocabulary classes at LSI schools.  For more information please visitwww.languagesystems.com