There will be 5 or 6 development cycles during the season in which skills will improve (or regress, for old players). The first one will be at the end of spring training.
Physical attributes only improve (or regress) during the offseason.
Injuries and injury recovery can affect both during the season.
Skills would include contact, batting splits, batting eye and baserunning for hitting, glove and arm accuracy for fielding, and control, pitching splits, GB and pitches for pitchers.
Physical skills would include power and speed for hitters, range and arm strength for fielding, and durability, stamina, and velocity for pitchers.