The bumps on the F and J keys on a keyboard are there to help the user find the "home" position on the keyboard.  That way, the hands are on the correct position on the keyboard.  

Users will put the index finger of their left hand on the "F" and the index finger of their right hand on the "J" before they type so they know where their fingers are located on the keyboard.

The "home" position for all of the fingers are "ASDF" for the left hand and "JKL;" for the right hand.