The Roman Legions used local coins to pay their troops. To make the coin usable as soldiers pay they would put a new image, a countermark on the coin. This one has two countermarks.

One countermark is a ship that is facing down (see the oars?). The other is of a pig which was the sign used by the Tenth Legion. (The pig is running)

Sometimes for a countermark they used roman numerals. In this case the coin was being used by the Tenth Legion so the roman numeral is an "X" for "Ten."