How to thanslate this sentence?

It means you should work only a few hours a day or a couple hours a week to keep familiar with the job so you don't get rusty or stale at it.

In America, "part-time" often means 20 hours a week (and "full-time" is 40 hours a week). As Intrepid indicates, it can mean as little as a few hours a day or a couple of hours a week, but the more common meaning of "part-time" is approximately 20 hours per week.

"Keep your hand in," can also allude to keeping contact with important people who can help you later, not just keeping your skills current (though I agree that's an important part of it).