What does cleaning your gun mean?

Answer 1

If we are not speaking of metaphors...

When a gun is fired, unburned combustion products accumulate in the barrel, along with small amounts of metal from the bullet.

A gun barrel needs to be cleaned after firing to prevent these from fouling the barrel rifling or blocking the chambering or firing of rounds.

In a revolver, the cylinder also needs cleaning to remove these products.

In the old days prior to smokeless powder and non-corrosive primers, the combustion products from ammunition were quite acidic and could cause a steel barrel to rust away from the inside.

The oil in guns also oxidizes over time into a sticky varnish and accumulates dust and dirt, requiring less frequent cleaning of the trigger and other moving parts.