Position plays a huge part of Texas Hold Em. Position can be split into 3 areas (im basing this on 10 people at a table):
The Early Position - This includes the 3 people after the big blind. This is the worst position to be in. It is very difficult to get any free cards in this position and often you will have to fold after you call a hand simply because there has been an additional 1 or 2 raises.
The Middle Position - The middle 4 people at the table.
The Late Position - The last 2 or 3 positions ie. the dealer position and the small and big blind positions. This is the best position to be in. You will see all raises which immediately gives you an advantage over other players. Not only that, after the other players have called you can raise again which means that they have to either call your raise or lose the right to any chips in the pot.
There are certain hands that you would fold if you are in the early position which you will call in the late position - simply because if you call it in the early position there is the likelihood that 1 or 2 people will raise would then make it a bad decision to play for the pot because the hands arent strong enough.
So in brief, the further you are from the dealer button the more information about you can gather to determine if you should call, raise or fold. Obviously your position could change depending on who is left in the pot - for example you were originally in middle position but are now in the late position because the players after you have folded.