Rep. Doug Collins tried to get Mueller to contradict his report by asking him whether "collusion" and "conspiracy" are the same thing. Mueller testified that they weren’t the same, but Collins cited a part of the report that he thought was contradictory.
On page 180 of volume one, the Mueller report said: "Collusion is not a specific offense or theory of liability found in the U.S. Code; nor is it a term of art in federal criminal law. To the contrary, even as defined in legal dictionaries, collusion is largely synonymous with conspiracy as that crime is set forth in the general federal conspiracy statute."
This part of the report was talking about collusion in the sense of corporate collusion, which is when companies conspire in an illegal fashion to help each other and hurt consumers.
That is completely unrelated to “collusion with Russia,” which has been the colloquial term used over the past few years to discuss potential cooperation between the Trump campaign and the Russian government.