How can an ADC measure below its resolution?
A simple example would be a regular voltage comparator, which will output 0 if below 0.5V and 1 if equal or above 0.5V. You can view it as a 1-bit ADC working in a range 0-1V.

Now consider a perfect input signal of 0.5V exactly. By our definition it will give 1 in the output. Now, we introduce a low amplitude white noise averaged at zero added to the signal. In this case the output will "jump" between 1 to 0 with probability of 50% at each point of time. If we sample this output over the time and then compute the average, we will eventually get a value 0.5 which is out of the original resolution we had with a single sample.

...And dithering is a method of introducing some artificial noise if the natural one is insufficient.