Output charger indications
It can imply a few things.

The 5V output will be guaranteed to be within tolerance (maybe +/-10%) when drawing a current of 1A from the device. That's about all that can be said without disecting the circuit in detail. For instance, at lower (or no-load) currents the 5V output might rise a bit to 6V or even higher. I would imagine this would be the case for a cell-phone charger.

Another example is the AC adapter for my yamaha piano - it says 12V on the side but off-load it's closer to 18V.

You also might be able to extract a little more than 1A as well but the voltage will probably be dropping towards 4 volts.

As I said before it's not guaranteed to mean much unless connected to the load it is intended.