The lithosphere is the upper most layer of the earth. It is divided into various tectonic plates. Those plates "float" around on top of the asthenosphere. The asthenosphere is not 100% solid. It flows around and has a consistency of something like silly putty. Because the asthenosphere is constantly moving around, the lithosphere's tectonic plates get dragged around as well. The plates have edges, and those edges are in contact with each other. Another name for those edges and points of contact is "boundary."
Plate boundaries interact with each other in three ways. The boundaries can slam into each other. That is called a convergent boundary. The boundaries can pull apart from each other. That is called a divergent boundary. Lastly, the boundaries can try to slide past/along each other. That type of boundary is called a transform boundary.