What do you use a clamp meter for?

The clamp part is for measuring amps via induction. You set meter to amps and clamp around only a single wire. Device you are measuring must be operating. This means you couldn't clamp around a lamp cord to a table lamp since the current induced in each direction cancels out. You would have to separate the wires. An electrician would carry a short extension cord with the wires already separated. Most often a clamp meter is use to checks amps in a breaker panel on the black wire coming from the breaker.