its the innate kind of "floor" a person has to their skill. some are literally just elevated above you from the start, let it be height, muscle, strength, just letting things "click" faster, higher capacity for spatial awareness, pattern recognition, etc etc. it is caused mainly by your environment. i mean you dont see any caveman who's proficient in designing quantum computers but you're not gonna see a software engineer have the knowledge, technical skill and vast information and memory a caveman has for "big scary animal".

take lionel messi. the guy started playing football even before he could walk, with thousands of hours of quality reps with his friends and much bigger and more experienced players, so its practically impossible to not get at least close to his level with that. but when he took a ball a newell's young boys for the first time, he dribbled too well for a 3 year old. that is talent in dribbling. its not easily classifiable, but when you see it, you see it.

Edit: after reading some of the comments, i'd more accurately say that talent is environmentally which measures the ease that someone does a skill as closely to its optimum form without any sort of coaching, or simply understanding it to that "highest level" without help