CC is constant current; cv is constant voltage. Typically there is a knob for Amps (to set CC) and a knob for voltage (CV).
The mode is dependent on the load and how these two knobs are set.
With no load, voltage knob adjusts the output voltage as you would expect. This is constant voltage (CV) mode. You can think of it as a limit: voltage setting sets maximum output voltage.
With a load attached, the current knob may influence the voltage. It sets a maximum current you wish to be delivered to the load. As the current approaches or exceeds the desired setting, the output voltage will drop in order to satisfy the maximum current setting. This is constant current mode.
In practice, it's quite useful for bringing up new circuits a bit more gracefully: you set a voltage, wind back the current, attach a load, and increase current until you're happy that no smoke is coming out
I'd recommend eevblog's notes on cheap lab supplies. I can loan my dual cv/cc supply to the space, much like my Tek540A it doesn't get much use any more