A digital signal has only two states. With digital electronics, digital signals are like a switch that can be turned on or off. When a digital signal is on, it can literally switch a small LED light on. It has power. When it's off, it has no power (or an extremely small amount of power) very much like an on or off switch.
The two states of a digital signal are often referred to as on/off, high/low, H/L and 1/0. They all mean the same thing. The term "high" means that the voltage level is high or fully on. The term "low" means that the voltage level is low or off. The H/L usage is just an abbreviation for High and Low. The digits 1 and 0 are where things really start to get interesting.
Did you know that the simple combinations of digital 1s and 0s can be used to represent all kinds of things? Math equations, the tones of a singer's voice, a map of the universe, the structure of a DNA. But a single digital signal can only be one of two things: 1 or 0; high or low; on or off.
Have you ever wondered what a 64 binary number really is? Well, it's a collection of 64 digital signals. When digital signals are used in this way, they are referred to collectively as binary and each signal is referred to individually as a bit. So each bit inside of a computer or microcontroller brain is actually a digital signal that can be one of two states: on or off.
When building robots, nearly all of the major circuitry will involve digital signals. The microcontroller brain will perform calculations and make decisions based on digital signals. You will wire up the robot controller's digital signal inputs and outputs to a variety of sensors, indicators and actuators. Every input and output will ultimately reach your robot's microcontroller brain as a digital signal.
It's truly amazing how much can be done with simple on or off signals.