Hi Stanley
Congratulations on digging up this little beaut!
It looks to me like Constans, who reigned from 337 to 353 AD and was the son of Constantine the Great, who was famously the first Christian Roman Emperor. The legend says "D N CONSTANS P F AVG".
We call the coin an "AE3", because we don't know what the proper names of Roman coins were at this time. Copper coins are just called AE1 to AE4 depending on their size.
The picture on the back is quite an interesting one. If you hold it so the letters "TR P" are at the bottom, it shows a Roman cavalryman galloping towards you, as if he's about to pass to your right, and spearing a fallen enemy cavalryman as he passes. It's a very daring design, especially for such a little coin.
The legend on the back always amuses me. It says "FEL TEMP REPARATIO", which loosely translates as "Happy days are here again". Only the Romans could depict a scene of slaughter on their coins and give it a legend like that, eh?
Mule