New players think about poker as one thing: the two cards in their hand. Better players think about three. Your cards are only one part of the decision. The other two, how many chips you have and where you sit relative to the action, change what those same two cards are worth.
This is the part everyone already thinks about: pocket pairs, suited connectors, big cards. It matters, but it's the weakest piece to rely on alone. The same starting hand can be a clear raise from one seat and an easy fold from another, before stack size even enters the picture.
Stack size changes what a hand is for. With a deep stack, you can call cheaply with a speculative hand like a small pair or suited connector. You're hoping to hit something big and win a large pot, and if the flop misses, you fold and have lost little. With a short stack, that plan stops working. There aren't enough chips behind to get paid when you hit, and every blind you pay while waiting eats a bigger share of what's left. Short stacks usually do better with simpler, more decisive plays: raise or go all-in with good hands, and fold the rest.
Stack size also sets a hard limit on what you can win. Once you're all-in, you can only win as much from each opponent as you put in yourself. Anything they bet beyond that goes into a side pot you have no claim to. See side pots explained for how that gets divided.
Position is information. Acting last means you watch everyone else check, bet, or fold before you decide anything. Acting first means the reverse: you reveal something about your hand before you've seen what anyone else does with theirs.
That advantage lasts the whole hand. The player in position gets to see every opponent's action first on every street, which makes it easier to control the size of the pot, bluff at the right moments, and get away from bad spots cheaply. That's why a hand that's a comfortable raise on the button can be a fold three seats earlier. Nothing about the cards changed. Only the order of action did.
No single factor settles a decision by itself. Each one shifts how much the others matter:
Reading a hand well means weighing all three together, not just checking what you were dealt.