Footprints are shapes on the ground where people or animals walked. The foot touches the ground and at that position a footprint appears afterwards. This can happen because the ground is muddy or snowy and the foot sinks a little bit into the ground. Another possibility is that the foot is dirty or has fresh color on it. When the foot touches the ground the dirt or color remains at the ground and marks the position of contact.
Footprints mark the way of people or animals. Hunters for example are able to follow an animal by following its footprints. Like that leaving footprints can be a big disadvantage.
In the time of internet people also talk about digital footprints. This are not real footprint but small bits of data (for example the IP address) which enable computer professionals to follow the way of an user in the internet. This can also be a disadvantage for the user who is leaving the digital footprints.