General Cycling Discussion - chain coming off

ok 3 times yesterday when i was going oh 15 - 20mph when i tried to go from the 2nd crank to the 3rd (i may be wrong on my terminology) so the 3 round things that are between the pedals, the chain chain came off... what can i do to fix that? ... maybe tweak the derailer some how? i dunno, i'm new, so i dont know anything, so be sure to talk to me like you would explaining it to a 5 year old, or i probably wont understand what your saying. :D

If the chain goes past the small chainring, it will come off and could get jammed. If this is what is happening, the limit screw on the front derailleur should be adjusted so it doesn't move the chain inward toward the seat tube and go past the small chainring.
Go here (http://www.parktool.com/repair_help/howfix_frontderailleur.shtml) for some good online help with pictures.

Good answer, Ron.

it comes off when i'm going to the biggest crank not the smallest one

The same idea applies except that you have to adjust the upper limit screw. The idea is that it physically stops the derailleur from going past a certain point.

Go buy a book on bike maintenance. We aren't likely to be able to help you unless we know which derailleur you have, etc.