Do you mean compared to other animals, or compared to other human species?

Either way, it boils down to the fact that homo sapiens could communicate about things not grounded in reality. Pretty much every animal communicates, but compare a lion telling another, "I saw a human near the big rock" [something factual, observed in the environment] to a homo sapien telling another, "I saw a lion near the big rock; it looked hungry. Maybe it is roaming around looking for food, so be careful" [the 'roaming around' was not observed].

In other words, homo sapiens can tell stories to one another. (Btw, a story isn't necessarily fictitious; the lion probably was roaming around. The important point is that the roaming wasn't observed).

Other species of humans were bound by tribe/blood relations. A neanderthal was bigger and stronger than the average homo sapien and would win in a fight; but neanderthals had tribes of (maybe) 50 or so people united by blood and pack (i.e you had to know each other) whereas homo sapiens could unite behind their common devotion to a sun god, or their shared love for a symbolic tree, or whatever. You can have two homo sapiens who don't know each, but they both believe in the same story. They can therefore unite, and all of a sudden when there's a dispute over resources, the 50 or so pack of neanderthals is in a fight with 250 sun god loving homo sapiens.

The fact that we can tell stories and have complex language means we can develop sustained collective learning. This kind of thing is cumulative of course, so over time our collective intelligence gathers more and more momentum.

TLDR: humans can communicate about things not immediately observed(/observable) in the environment around them. Communicating this allows for collective sustained learning. Like a rolling snowball, this learning just gets bigger.