It was the assignment operator in Algol, which along with Fortran and Lisp was one of the most influential programming languages for decades. Its basic syntax was the prototypical "psuedo-code" in textbooks for decades after that.

The operator was inherited by Pascal, Ada, Delphi. That line of language syntax died off in the late 90's though, so I can see why younger (and in particular self-taught) programmers wouldn't be familiar with it.