Position — where you sit relative to the dealer button — is the single biggest preflop edge in poker. Acting later means acting with more information, so the later your seat, the wider you can play. Here is every seat, its job, and roughly how wide to open, each linked to its full range chart.
First to act — the whole table is behind you. Tightest opens.
A step later; open a touch wider than early position.
First of the prime late seats — open wide, attack limpers.
Act last on every street postflop — the best seat, widest range.
Post half a blind, then act first postflop — the worst seat. Raise or fold.
Already posted a full blind, so defend wide vs steals at a discount.
The player who acts last sees what everyone else does before deciding — a permanent information edge that compounds on every street. That is why the button is the most profitable seat and early position the least, and why good players open far more hands on the button than under the gun. When you can't have position, you tighten up; when you have it, you widen and apply pressure.