The Physical Layer is concerned with the actual communication of single bits of data from one point to the next by some physical means.  In the case of fiber optic communications, this is accomplished through pulses of light.  Other forms of wired communication will use an electrical current that alternates between some higher voltage and a lower voltage to indicate whether a 0 or 1 was transmitted.  Consumer-grade wireless networks use radio signals to transmit data.   The physical layer is also concerned with detecting and recovering from errors that may have occurred.  For instance, consider what happens if the voltage of the electrical current is at some intermediate level, instead of being either high or low.  It is the responsibility of the physical layer to recover from this error.  The physical layer does not know how to interpret the data that is being transmitted – that is beyond the scope of its responsibilities.  The physical layer can be thought of as point-to-point communication.