forceing apps closed

[toadfish]

What does this mean? I am new and really have no idea what it does to the app and the phone.
Are there some apps that you can force close and not effect the phones email,txts etc? Are there any that should never be closed? Are them some you can just closed and forget? I use the GPS alot and normal to med usage every day. Just did not want to start closing things that would harm the phone. Also want to make it run as smooth as possible with out rooting it.. What if you Close a app and then need to use it. Will it start running again when you tap it or do you need to go to setting of apps and reopen?