A pawn that reaches the far end of the board doesn't stop, it transforms. You must replace it with a queen, rook, bishop, or knight of your color, and nearly always you want the queen, the strongest piece on the board. This is why endgames revolve around pawns: each one is a potential new queen.
You can promote to something other than a queen ("underpromotion"), and once in a while it's right, usually promoting to a knight for an immediate fork or check, or taking a rook to avoid an accidental stalemate. But those are rare exceptions. Queen first, ask questions later.
8
7
6
5
4
3
2
a1
b
c
d
e
f
g
h
White's pawn stands one step from the end of the board, escorted by its king.
Promote the pawn to the strongest piece.
8
7
6
5
4
3
2
a1
b
c
d
e
f
g
h