Jewish leaders did not think that it was appropriate to use Roman money to pay for items in the Jewish Temple so there were money changers who would change your Roman coins into Temple script that could be used to buy items in the Temple. These are the people that Jesus was upset with when he entered the Temple. Scholars aren’t exactly why Jesus was upset but it is thought that he was upset because they were making money on the transactions.

(The Great Courses – How Jesus Became God)