WELL HEY BACK TO YOU! Freewheel or single speed is where you can idle your feet. Fixed is where you can't, meaning your feet are always moving along with the crank and pedal as it revolves.

Don't get a flip flop hub; get a fixed/ fixed hub if you can/ willing to pay a little extra. You can put a freewheel on both sides and a cog and lockring on both sides as well. Also provides some insurance if you happen to accidentally strip one side on accident.