Understanding DSCP Values and Marking

I am currently configuring Lync for our enterprise. I have it running but I want to implement QOS so that it has priority over certain data types and less than other.
These are the port ranges.

Audio      49,152 – 57,500
Video      57,501 – 65,535
Application Sharing      32,768 – 49,151
Client File Transfer       24,733 – 32,767

Now, in QOS policy, it asks for a DSCP value for each. How do I know what value to use and what it means?

If someone can explain the significance (Ive read articles but still dont really understand), it would help me out a lot.

Thanks