The accumulator.

An accumulator is a tank spliced into the suction line, between the indoor coil and the compressor. Refrigerant returning from the house enters it, and anything still liquid falls to the bottom instead of carrying straight on.

Gas is drawn off from the top. Liquid sits and boils off gradually, metered back in slowly enough that the compressor can cope.

Why it is there

A compressor pumps gas. Liquid does not compress, so a slug of it arriving at speed can do real mechanical damage, and a steady trickle washes oil out of the bearings.

Heat pumps almost always have an accumulator because running in reverse makes liquid return far more likely, particularly during and after a defrost cycle. Straight cool systems sometimes have one and often do not.

What it does not do

An accumulator is a safety margin, not a cure. If your coil is freezing, the accumulator is the reason you have a working compressor rather than a ruined one, and the freezing is still the problem to fix.

It also has limits. Sustained liquid return, which is what running a badly iced system produces, will get past it.

When it is the fault

Rarely. They are simple vessels with no moving parts. What does happen is corrosion and leaks at the connections, or an internal oil return hole plugging, which starves the compressor of oil.

Both are refrigerant circuit work, which means recovery, repair, evacuation and recharge, and that lands past the one thousand dollar ceiling that applies here more often than not.