world_to_map() converts from global coordinates to tile coordinates. Say your tiles are 64x64 and
KinematicCollision2D.position gives you (322, 520).
world_to_map(KinematicCollision2D.position would return
(5, 8) - you're standing in tile (5, 8). If you need to know the tile below you, for example, would be
As I read your question, that's what you were looking for. If you need to know what tile you hit,
TileMap.get_cellv(Vector2(5, 9)) is going to give you the ID of the tile in that position.