When you send a app (ex. myspace) to your background on a rumor touch, what does that mean and what does it do?

i have a rumor touch and i want to know when you send a app. to the background what does that mean or do? all i see it does is that there's a +1 at the top of my phone... its weird and i wanna know what it means? help me plz :-/When you send a app (ex. myspace) to your background on a rumor touch, what does that mean and what does it do?
I believe that it hides it so that you can do other things on the phone, like play music and still text at same time. The +1 tells you how many applications you have running at the time.