Signals are the one important thing about BCIs. They are used to tell the prosthesis what to do. Signals represent nerve cell activity in certain parts of the brain under given circumstances. These include EEG-rhythms that reflect oscillations in neural circuits of certain areas of the brain's cortex (more).
Signals can also be so called evoked potentials which are visible in the EEG as response to stimulation, for example visual stimulation (more). Finally, action potentials (more) produced by particular neurons can act as signals. The latter ones are usually not recorded by an EEG but by implanted microelectrodes that are in direct contact with brain cells.
It is important to realise that although nerve cells naturally communicate with muscles and the rest of the body, the sampled activity that is recorded from the brain's cortex by either EEG or electrodes is not the way the brain usually talks. In other words: Signals reflect brain acticity rather than being brain activity themselves. This has some deep theoretical impact. (more)