Hub compatibility for Felt Nine 60 2017
135 means 135mm rear drop-out spacing - the older standardard for MTBs that is still used on less expensive bikes.

QR is quick release, as opposed to a solid bolt on axle or more modern through axle.

6-bolt refers to the disc mounting standard, the other being Shimano's Center-Lock.

The free hub body width needs to be compatible with 8/9/10 speeds cassettes. You'll obviously need a hub with the same spoke count.