Information is encoded by changing the signal that is being sent. The amount of data you can transfer per second is a function of how often you can change the signal being sent, and detect the change on the other end. We can do this really, really, fast.

Once you've reached the limit of your ability to make the signal change more often, you can increase the amount of information transferred by making each unit of this signal contain more information than just "on" or "off". Instead of a signal where everything above a certain amplitude is a 1, and below it is a 0, you can split the amplitude into more sections, each one encoding multiple binary digits. You might split it into four different amplitudes corresponding to 00, 01, 10, 11.

You can build more and more advanced ways of encoding more information into the signal. For example, you can also send multiple signals of the same wavelength that are out of phase with each other, and if you have two signals, and one encodes phase angle and the other amplitude, now you can combine those signals to create a "line" that points to a location in 2D space. Then you can construct a grid of possible combinations of bits, 0000, 0001, 0010, 0011, etc. in that 2D space that each unit of the signal "points" to.

On top of that, you can send multiple different data streams through the same fiber at the same time by using different wavelengths of light for each of them.

And then you have many, many fibers in the same cable.