The term kit lens will come across quite often during your search for lenses as well as when you photograph and when you take part in discussions related to lenses in particular.
Kit-lens refers to a lens that is paired with a camera when you buy one. This is done by the manufacturer when they release a new camera. The lens is selected depending on a number of parameters. These include the camera, the format and the intended user among other things.
The cheapest kit lens is the ubiquitous 18-55mm wide-zoom. This lens was designed to cover the most essential focal length for everyday photography requirements. This lens is paired with entry-level crop DSLRs such as the Nikon D5100, the Nikon D3300, and the Canon Rebel series cameras. Another very popular kit lens is the 55-250mm one, especially the one manufactured by Canon. This lens is sometimes bundled with the 18-55mm as a set of twin kit lens bundle.