Definition of "in position
When we are "in position" in poker, it means that we talk last after the flop. Preflop, being in position means being on the button or cutoff. The worst position is the small blind who will never be in position on any other player at the table. You can still be in position after the flop. For example, a player in UTG position is not considered "in position" preflop. But if he is heads-up on the flop against the small blind, he is considered to be in position because he will be the last to speak on all streets.
The advantage of playing in position
The beauty with playing in position is that even if you don't understand why position is important, you will still benefit from position. Position means to speak last. Speaking last gives us an advantage over other players because we have more information than they do. You are UTG (out of position) and have a pair of 9's. You open your hand to 3 big blinds. You then get 3bet by the player in UTG+1 position and 4bet by a conservative player in MP. The small blind pushes all in and the call goes to you. This is obviously a fold and you will have lost 3 big blinds. If you had been in position on the button with the same hand, you would not have lost those 3 chips.