Just as multiple levels can be used to send multiple bits at a time on a digital signal, multiple amplitudes, frequencies or phases can code groups of bits. Here's 1011001101011110 using four phases, 45, 135, 225 and 315 degrees:

A signal can be modulated with multiple values of each of the three parameters, allowing more than one bit to be sent at a time. Here we send 1011001101011110 using two magnitudes and two frequencies. This gives four combinations, allowing us to send two bits at a time.

Here we have four phases, 45, 135, 225, 315, and two amplitudes to send 3 bits at a time, sending 001101011100010011.

And you can try some more, here.