To paraphrase means to restate someone else’s ideas in your own language at roughly the same level of detail.
Whenever you paraphrase, remember these two points:
You must provide a reference.
The paraphrase must be in your own words. You must do more than merely substitute phrases here and there. You must also create your own sentence structures.
Finding new words for ideas that are already well expressed can be hard, but changing words should not be your chief aim anyway. Focus, rather, on filtering the ideas through your own understanding.