Glossary
Counter-strafe
In one line: Stopping instantly by tapping the key opposite to your movement, so your shot is accurate.
What it means
In VALORANT bullets are only accurate when you are still (or almost still). If you are moving right with D and let go, your agent slows down but does not stop at once. Counter-strafing is tapping A (the opposite key) to stop dead and shoot right after.
It is the basic movement of every duel: move to peek, stop, shoot, move again if needed. Players who shoot while moving spray bullets everywhere, even with good crosshair placement.
An example
You are behind a wall and want to check an angle. You press D to swing out, and the moment you see the enemy you tap A: you are standing still, your first shot is accurate and it lands on the head. Then you go back behind the wall with A.