There are many kinds of connectors that crimp onto a wire. These connectors serve a wide variety of purposes, which all relate to joining wires together to make circuits.
A crimp-on connector has an end of a wire inserted into it, and then you crush the connector in a specific way so that it grabs the end of the wire. Often one uses a crimping tool, but in some cases pliers can do the job. In some cases there are special purpose crimping tools for specific connectors.
Stripping wire is a first step to building an electrical cable or any other electrical connection. Wire almost always has insulation around it to keep the wires from getting crossed. To make a connection between two wires one must join them together, or attach connectors to them and attach the connectors together. The first step is to prepare the ends of the wires by stripping off the insulation.