Crimping refers to the fitment of the lugs on to the wire or the conductor. Crimping is a very important part of wiring. Poorly crimped conductors can cause loose contact, arcing and malfunctioning in the circuit.
Crimping is done by deforming the lugs to suit the circumference of the wire. 
Crimping can be done manually using special hand-tools. There are also dedicated special crimping machines for larger cables and conductors.
There are also bench mounted Crimping devices. These devices are pneumatically operated.