BTU is a measurement of heat/energy
Watt is a measurement of power (energy/time)
A BTU is similar to a joule, a BTU/hour is similar to a watt.

When an AC unit says 5000 BTU it means that the unit will remove 5000BTU/hour from a space, though this depends on the temperature difference.  It is not a measurement of how much power the unit is drawing.  That would depend on the specific unit you are looking at.