Position
Position is where you sit relative to the dealer button, which decides the order you act in each betting round — acting later is a real advantage.
In Texas Hold'em, the button moves one seat to the left each hand, and your seat relative to it sets when it's your turn to bet, check, call, raise, or fold. Acting after your opponents is called being in position; acting before them is being out of position. The later you act, the more information you have, because you've already seen what everyone ahead of you did.
It matters on every street. Pre-flop, seats run from early (like under the gun, first to act) to late (the cutoff and the button), but the blinds actually act last pre-flop since they've already put money in. On the flop, turn, and river it flips: the player on or closest to the button acts last in each round. That post-flop button seat is the most powerful spot at the table.
For a beginner, the simplest takeaway is to play more hands when you're in late position and fewer when you're early, since acting last lets you control the size of the pot and bluff or value bet with better information. You don't need to memorize charts yet — just notice where the button is before you decide how loose to play.
Example
You're on the button holding 9-club 8-club, and everyone checks to you on a flop of K-spade 7-heart 2-diamond. Because you act last, you've seen that nobody showed strength, so you can fire a small bet (say 3 chips into a 6-chip pot) and often take it down — an option the early-position players never had, since they had to act before seeing anyone else.