A couple of things:
1) It starts (or adds to) their ML service clock, but only for the "in-season" days they are actually on the ML roster. Post-season days do not count.
2) If they were not already on your 40-man roster, they will automatically be added to the 40 (if you have room), and their option clock starts ticking. If they were already on the 40, then their option clock already was ticking.
3) Their salary will increase to $327k if they were on a minor league contract. As long as you demote them back to the minors before roll over to the next season, they will roll to next season on a minor league contract and their salary will revert back down to 55k. But if they stay on the ML roster at roll over, they will roll on a major league contract at $343k for next season.