The eyepiece is placed near the focal point of the telescope to magnify the image projected by the objective. It is the closest element to the eye when the subject is observed through the telescope. The amount of magnification depends on the focal length of the eyepiece. It is comprised by several "lens elements" in a housing, with a "barrel" on one end. The barrel is shaped to fit in the telescope focuser.
A high-quality eyepiece makes the targets you are observing, brighter, clearer, and more enjoyable.