A "Hit Marker" is an icon that appears around your crosshair when you hit something. There are a few things that will produce Hitmarkers on your screen when you shoot them, stab them etc. Zombies, players, vehicles, trees, player made buildings/structures, and sentrys
You can change your hitmarker's colour, as well as your critical hitmarker.
The difference between a regular hitmarker and a critical hitmarker is a regular hitmarker appears when you shoot someone in the body, leg, arm etc. but when you shoot them in the head, a critical hitmarker appears. The hitmarker is white by default and the critical hitmarker is red by default. This also applies to zombies. The colour of critical hitmarkers change the colour of glowing sights/scopes (dot sight, halo sight, chevron scope, etc.)