I believe it means you have an Analog Output with 8 bits of resolution.
8 bits means that your full scale of output is either -128 to 127 (Or is it -127 to 128?) or 0 to 255. The more resolution it has, the finer tuned the analog signal can be. 10V/256 (8 bits) is less resolution than 10V/65536 (16 bits).
Edit:
Just to explain where those numbers came from. The bits are how many bits of binary are used for the output.
8 bits of binary is 2^8 = 256.
16 bits of binary is 2^16 = 65536