1.
one's skill improves; to get better at
To improve at a skill or craft through practice, such as cooking, a sport or a trade. 腕 here means skill.
腕 means skill here, as in 腕がいい (skilled) and 腕を磨く (to hone one's skill).
Common patterns
- 〜の腕が上がる: to get better at ~
- 腕を上げる: to improve one's skill (transitive; also praise: 腕を上げたね)
- 腕が落ちる: one's skill declines
Watch out
The literal meaning also exists: 肩が痛くて腕が上がらない (my shoulder hurts so I can't raise my arm).