Gi,
Digital: just means "clear states". Like ON/OFF or FORWARD/STOP/REVERSE. Or: 1, 2, 3, 4...
In opposite analog: it means there is a range. Like car battery voltage from 10.5V to 14.4V. Or "soft" statements like cold and warm, where no clear definition exists.
Analog values continously change. If you measure room temperature now...it will change the next moment, even maybe just by 0.0001°C
Signal means "information". We - in this forum - often think of electrical signals, like current, voltage, frequency...but a signal may also be optical (IR remote control, or even the signal of a lighthouse. Signal may also be accoustic, like musik, speech, ultrasonic sound, a buzzer..
Klaus