Probably a knot that has almost grown over.
I would try to excavate to see if there is a cavity of black manky stuff beneath it. But try not to cut into the sapwood, an 1/8" drill in a hand drill or a ground down needle file is good for picking it out.
It doesn't matter too much in terms of the integrity of the back, but what you want to avoid is a cavity that goes deep into the compression side which will collapse and cause a pinch on the belly. Is there any sign of a corresponding pin knot on the belly.
Hopefully it will only go as deep as the sapwood in which case, fill it with what ever takes your fancy, just to keep any moisture or rot out of it. Filling won't strengthen the back.
If you are really worried by it you could rasp out a long shallow scoop say 3" long down to half/threequarters the depth of the sapwood and clean it out, fill it and put a matching sapwood patch over it, but I don't think it's necessary.
They are just nice character features... if you did patch, you could patch over the little cluster of pins with the same patch.
What draw weight is the bow going to be?
Del