First of all, this sentence is casual. It could be used between friends or to someone who is younger than you are.
적당히 means 'moderately', 'appropriately' or 'suitably'. 좀 is short for 조금 meaning 'a little'.
'해' means 'do' as an imperative form.
"적당히 좀 해." literally means "Do it a little appropriately!"
That would be a very similar expression such as "Take it easy!" or "Go easy on it!" in English.
However, "적당히 좀 해." might sound a little offensive depending on the situation or tone of the speaker.
If it is used in a bad situation, it would have the meaning as "Cut it out!"
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