The hard start kit.

A hard start kit is a start capacitor and a relay wired across the compressor. It gives a much bigger shove at the moment of starting, then takes itself out of the circuit once the motor is up to speed.

It makes a compressor that is struggling to start, start.

When it is the right call

An older compressor with worn bearings genuinely draws more to get going than it used to. On a long or undersized run of wire, voltage sags at the moment of start. In both cases a start assist is a legitimate and cheap way to buy real time on equipment that is otherwise working.

It also reduces the moment of very high current draw, which is easier on everything upstream.

When it is being sold to you

Here is the part worth being blunt about. A compressor that will not start is usually a dead run capacitor, not a tired compressor. Fitting a hard start kit on top of a failed run capacitor can mask the problem well enough to get through the visit, and the system comes back.

A hard start kit also does not fix: low refrigerant, a fouled condenser coil, a pitted contactor, or low voltage from the utility. If one of those is the actual cause, the kit buys weeks.

So the question to ask is what was measured before it was recommended. Microfarads on the run capacitor, amp draw on the compressor, and voltage at the unit under load. If those numbers were not taken, the kit is a guess.

What it should cost

It is a small part and a short job. On a quote where it is bundled into a much larger number, it is worth asking for it line by line.