6 - type code: single-row, deep groove ball bearing
2 - series / robustness: (2 corresponds with 'light', which is more durable than 0, but less durable than 3 or 4)
01 - size code: 12mm bore (inner diameter)
RS - grease seal: rubber seal one side (no seal would be '6201', both sides sealed would be '6201-2RS'. Z and ZZ are used similarly to indicate presence of steel shield(s))
The 6201 is a good fit for this application, and providing the gerbil wasn't too hard on it, should last a long time.
I recently took apart my weed whacker and it used the same ball bearing. I actually learned that these ball bearing casings (?) are super common. I replaced mine with a skate board bearing and it's as good as new.