An easement is the right to use someone else's land. If you come home to find your neighbors having a party in your backyard, perhaps they have an easement to use it — although most likely they’re just rude.
If someone has an easement on your land, they have a right to use or cross your land without your permission. For example, there may be a plot of land the military needs to move troops across. With an easement, the military can cross this land legally, even though the owner may not like it. Evidently, this practice has been going on for centuries; scholars think this word has been in use since the 1400s.