This is to help the player see where they are in relation to the area that the map is showing.

When placing a map into an item frame, the map displays with a green pointer shown at the location of the item frame. Other players are displayed on the map only if they have a map in their inventory cloned from the one being looked at. If a player holds a map whose clone is on display in an item frame, then that map updates while holding its clone. The world is recorded as-is during exploration, meaning that if the world is modified, a player must revisit the area while holding the map to update the map's view. To record the world on a map, that specific map must be held in the player's hands while the player moves around the world. This way, no two maps of the same size can ever partially overlap and every map can display only a fixed area. To make a map that is not identical to the first one, the player would have to move outside of the edges of the first map (because then they would be in a new grid square). For example, if a player uses a new map in a certain grid square, and then moves a distance away and uses another fresh map but is still within the same grid square, both maps appear identical. The map does not center on the player when created, rather, the world is broken up into large invisible grid squares, and the map displays the area of whichever grid square it is in when it is first used.

A pointed oval pointer indicates the player's position on the map, and moves in real-time as the player moves across the terrain shown on the map. After conversion to a drawn map item, it starts to draw a top-down view of the player's surroundings, with North pointing to the top of the map. This map can then be adjusted to different zoom levels.

The map is drawn for the first time when it is held and used (with use item).

Minecraft maps city 1.12