Being in position in poker is simply acting last in any particular street. For example when your on the button you act last on every street other than pre-flop, but for the rest of the hand you are in position because all other player have to act before you giving you the positional advantage in getting to see what your opponents do first.
It is such a massive advantage when in position, so much so must of your profits in poker come from being in position. This is why good players are aware of positional advantage and open up their playing ranges the closer they are relative to the button. Good poker players are also watchful of the hands they play in early position, that is to say the further away from the button they are and the more players left to act behind them because they know they will have to play the future streets out of position, and play fewer hands.
It is not uncommon for good profitable players to raise up to 80% of their hands (sometimes more) when folded to on the button in a typical no-limit hold’em game because of the power position gives them. They understand it’s not just about the value of the hand they are holding but the power that being in position for the rest of the hand gives them. Conversely you will lose money in the long run by being out of position regardless of how skilled you are, you will lose money playing out of the blinds there is just no way around it, that’s how powerful position is and that’s why it’s so important to understand it.
For any serious student of the game understanding position and how it effects your strategy and win rate is essential in developing your poker skill set.