Hi Peter,
Welcome here and I wish you a lot of fun 
An FX Loop is essential an insert point in your amplifier circuit that allows you to insert a piece of gear after the preamp circuit, but before the power amp circuit…
A lot has been written about it here and for a clear explanation you will have to wait for someone else, but you can simply connect a looper to the 50… I have 2 amplifiers here and the one I use does not have an FX loop and the looper works fine for how I use it… but you have to separate those almost identical names…
Maybe this topic helps to clear something to see the different?
Greetings ,Rogier