GPP: General physical preparedness, aka "fitness."
HSPU: Hand stand push up. Kick up into a handstand (use wall for balance, if needed) bend arms until nose touches floor and push back up.
HSQ: Hang squat (clean or snatch). Start with bar "at the hang," about knee height. Initiate pull. As the bar rises drop into a full squat and catch the bar in the racked position. From there, rise to a standing position.
K2E: Knees to elbows. Similar to TTBs described below.
MU: Muscle ups. Hanging from rings you do a combination pull-up and dip so you end in an upright support.
OHS: Overhead squat. Full-depth squat performed while arms are locked out in a wide grip press position above (and usually behind) the head.
OTB: Over the ball
PC: Power clean
PR: Personal record
PP: Push press
PSN: Power snatch
PU: Pull-ups, possibly push ups depending on the context
Rep: Repetition. One performance of an exercise.
RFT: Rounds for time
RM: Repetition maximum. Your 1RM is your max lift for one rep. Your 10 RM is the most you can lift 10 times.
SDHP: Sumo deadlift high pull
Set: A number of repetitions. e.g., 3 sets of 10 reps, often seen as 3x10, means do 10 reps, rest, repeat, rest, repeat.
S2O: Shoulder to overhead.
T2B: Toes to bar. Hang from bar. Bending only at waist raise your toes to touch the bar, slowly lower them and repeat.