HELP! How to "close" a mesh? w. image

 HI! I need to figure out how to "close" a mesh so you don't see the deleted body under. Example how to close a skirt so you can't see up when you tilt the camera (but in my case a shirt)

I have an example of an EA mesh that is closed imgur.com/t2o1Fkb


I have googled away and can't find any tutorial about it, I'm not even sure which words to google, so I would love some help!