An easement is a legally binding rule about the use of a property by a someone who doesn't own that land.
There are many different kinds of easement, separated into two broad categories: affirmative and negative. Affirmative easements preserve nonpossessory rights to use land in certain ways, such as passing through an otherwise private space. Negative easements restrict the use of a property by an owner, limiting certain kinds of construction or landscaping.
A property subject to an easement is known legally as a "servient estate," while a person or entity with a positive right granted by an easement is called a "dominant estate."
Easements can be established by explicit provisions in a written document like a deed, and can also arise over time from habitual use. If people cross a piece of private property to get to a beach, for example, and that behavior continues for several decades, this might create a public right of way.
It's a good idea to establish whether your estate has any easements attached before you start any construction projects, especially along property lines.