An easement is the right to use real property without possessing the land. A property owner can grant someone else an easement for limited purposes.
Easements are granted for a number of reasons to grant access to non-owners of the property. A neighbor may grant an ingress/egress easement to their neighbor to allow them to use a sidewalk or driveway that passes over their land. A homeowner may also grant easements to utility companies to allow them access to underground or overhead utilities that pass across their land.