That kind of growths are typically called "galls" Usually there is some kind of insect that lay egg inside the leaves that causes them. My oak tree gets them from tiny little wasps, but the damage is not significant enough to harm the tree.
I don't know what kind of tree or insect is involved in your situation, so the extent of the damage may be different and you may need to control it. I have a Liquidamber tree that I am hoping to turn into an bonsai, and when that tree developed leaf galls, I just clipped off and destroyed the affected leaves.
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