The resolution of a A/D converter means the accuracy, and it ranges from 8 bit and higher, the more bits, the more accurate results you can get
8 bit resolution has a maximum decimal value of 255, which means you can measure 255 different voltages ranging from 0 to whatever reference voltage you are using
16 bits has a maximum decimal value of 65535, big difference in accuracy, but only twice the storage needed
Say, if you wanted to make a voice recorder, and you needed the top sound quality, you need to get better resolution because it will make the sound so much more accurate, but the file size will be bigger