PO in baseball most often stands for putout, the credit given to the fielder who actually records an out, such as catching a fly ball, tagging a runner or stepping on a base. In recruiting, PO means pitcher only, a player who pitches but does not hit. Some box scores also use PO for a pickoff. The chart below explains all three meanings, which positions record the most putouts and the related abbreviations.
Where you see it: Box scores and fielding statsCredit to the fielder who actually records an out: catching a fly ball, tagging a runner, stepping on a base for a force or catching strike three.
Where you see it: Recruiting and showcasesA player who only pitches and does not hit or play the field, common in high school and college recruiting.
Where you see it: Some box scores and play by playA pitcher or catcher throws out a runner caught off base; many scorers use PK instead.
Putout vs. Assist
On a typical groundout to the shortstop, the shortstop gets an assist for fielding and throwing the ball, and the first baseman gets the putout for catching it and touching the base. On a strikeout, the catcher usually gets the putout. A fielder who makes an out alone, like a first baseman fielding a grounder and stepping on the bag, gets the putout with no assist, scored as unassisted.
Related
Baseball scorekeeping symbols · Force out vs. tag out · Baseball positions and numbers
Official definitions of putouts, assists and errors are in the MLB.com glossary.