You've heard that your iPhone or iPad will run faster and longer if you close apps you are not currently using. Wrong! Doing so can actually be counter-productive by slowing your use of the device and by hurting battery life when iOS has to restart that app the next time you use it.
Closing iPhone or iPad apps can help fix a frozen app, but in general, just leave it alone and let iOS manage everything. This terrific article on the How-To Geek blog debunks this myth in great detail.