What is signal resolution?
To keep it simple: resolution is a number of steps an ADC can produce.

Example: 8-bit resolution means that the ADC can sense 256 levels (2 to the power of 8). If the reference voltage is 1V it means that the ADC can roughly sense a voltage difference of 1/256 V = 0.0039 V.

Another analogy: it is the number of shades of gray the ADC can see.