I’ve always been under the impression that it’s a metaphor for a spider’s many eyes, allowing the spider something approaching 360 degree vision. This would be a suped up version. Otherwise, his mere athleticism doesn’t really explain him dodging bullets. Though…other superheroes with less athleticism and no spidey sense do it all the time.
I definitely recall reading a spidey comic where they tried explaining it as a sort of limited precognition. I think it might have been a character like Destiny or something that tried to explain that Peter instinctively senses the future a few moments forward and then all the permutations and probabilities come back to inform how it helps him do exactly what is necessary to avoid the danger.
However, i just tried googling to find any canon explanation of his spidey sense and came up blank on the first page of results. I got a whole lot of hits…just none telling me a canon explanation. Though two did specifically say that the power is ‘deliberately vague’. So it’s possible that my remembered explanation was from something non-canon or was just my memory failing me.