To assign another meaning to a symbol that already has a meaning, or a new operation to an operator that is already assigned to another operation. Perhaps the quintessential example of overloading in mathematics is the case of the Greek letter $\pi$. In geometry $\pi$ refers to the ratio between the perimeter and the diameter on a circle, while in number theory $\pi(x)$ refers to the prime counting function, and not the multiplication of $x$ by the circle perimeter/diameter ratio.
In some cases it is possible to resolve meaning purely from context. For example, if $i$ occurs under a $\Sigma$ or a $\Pi$ it is most likely just a generic iterator. Absent those Greek letters, it could be the imaginary unit, $\sqrt{-1}$.