How much power does 5V and 6A provide in Watts?
If it a power supply, then it means that you can draw a max of 6A at 5V (i.e. 30W). If it is a device which requires electrical power to operate, then it means that the device requires 6A at 5V to operate (again, 30W).

Note that if it is a 5V power supply, 6A is the max amperage it can provide. The amperage it actually provides to your circuit depends on the circuit's power requirements. So if you hook up a microcontroller to the power supply, for example, it would only a fraction of 1A.