As it's rather difficult and long-winded to describe the functioning of SSVEP and p300-based BCIs using written language, I've included these videos of other people's projects as examples [skip to fourth to see a robot!]:

In this first video, an SSVEP-based speller is shown. As you can see, each letter flickers at a different frequency..

In this second video, an SSVEP-based cursor is shown. As you can see, each direction choice flickers at a different frequency..

In this third video, a p300-speller is shown..

In this fourth video, somewhat more excitingly, Nao is controlled by SSVEP. The user attends to the flickering lights on the frame which correspond to which of the four commands they desire to send to Nao (e.g. walk forward)