The original literal meaning of "threshold" is the board at the bottom of a doorway. You have to step over the threshold in order to get into the room.
So, "threshold" also means "some value you have to exceed, in order for something to happen."
"Felony" is a technical legal word for "serious crime." Your passage indicates that there is a "dollar threshold" for "felony fraud." It might be, let's say, $1,000. If you defraud someone of $500, or $999, you have "broken the law," but it is only a "misdemeanor." If you defraud someone of over $1,000 you have committed the crime of "felony fraud."
Figuratively, the fraud needs to step over the $1,000 "threshold" in order to be considered a felony.