Regenerative braking: whenever the car is being retarded - to slow down, to maintain speed downhill, etc., the electric motor is used as a generator, to retard the driving wheels and thereby place some charge into the traction battery. You don't choose this - the car does it for itself.
There are conventional brakes as well. The car will decide which to use depending on (for example) how hard you push the pedal.
And then, when you (for example) set off from a standstill, that stored traction battery power is used initially to start the vehicle rolling. Thereby saving some fuel.
Makes for a quicker getaway whilst allowing the engine to be stopped rather than wasting fuel idling. Typically, regeneration alone only stores enough power for a few yards of traction.
That's true for all hybrids. The plug-in element of this type of car adds a bigger traction battery that can be externally charged hence allowing for an initial mileage to be done entirely on battery power.
And yes, when charging, it will only draw power whilst there is spare capacity in the battery. When it's full, it will stop.