It used to be easier in an earlier incarnation of the forum. It kind of got lost in the shuffle switching to the current format.
If you like what someone said our if they went out their way to help, you give them karma. Likewise, I'm pretty sure you could take it away. I believe it was intended as a way to gauge a person's reputation in the forum at a glance.
I pretty much exclusively use the mobile version of the forum and haven't even seen the karma thing for quite some time. I think it's still functional on the desktop site, but I don't think it's prominently displayed.
"Ground cannot be checked with a 10mm socket"
The following user(s) said Thank You: juergen.scholl
It's a fun feature! But it also kinda overlapped with the Thank You's. The etiquette for each was never really ironed out before we transitioned to the latest format...
The following user(s) said Thank You: juergen.scholl