An easement does not mean someone else owns your property. It just means that they have a right to use it in a specific fashion. The property is still yours, even though the easement may limit your use of that property in some way. Typically, the easement is just part of that property. If you sell the land, the easement generally goes with it.
One of the most common examples is an easement that grants someone else the right-of-way and the ability to cross your property when needed. Legally, you cannot tell them to get off of your property, as long as they are using it properly within the confines of the easement, even though you own it.
This type of easement is used when one piece of property blocks another one from access to the road, as may happen if a lot gets split in two. You can sell that back lot, but the person who buys it may need an easement so they can use your driveway or a road across your front lot to get to the street.
The big benefit of an easement, for you, is that you do not have to sell part of your front lot along with the back one. With no street access, it’s likely that no one would buy it, after all. The easement means you get to keep the land you want and sell the land you don’t, but it still makes the property useable to those potential buyers. The alternative would be dividing the lots in a different fashion so that they both reach the street, which may not be what you want to do. The easement means you don’t have to take that step.
If you’re buying or selling a lot with an easement, you absolutely need to know what it means for your property rights.