Generally that means it's the maximum you can tow. So it would be the trailer with everything you load into it.
A couple of caveats:
1) Sometimes there is also a maximum tongue weight
, which can limit you regardless of the max trailer weight.
2) With some vehicles, the vehicle's occupants and luggage have to be subtracted from the total allowable weight.
3) Some vehicles also have two different weight limits: With trailer brakes
and without trailer brakes
This should all be in your owner's manual. That's the best place to find the info for your
specific model (sometimes even a given car model can vary depending on options, year, etc. so don't just go by someone else with your same type of vehicle).