From Liquipedia StarCraft Brood War Wiki

Multitask is the ability to manage multiple situations that happen at different locations across the map at the same time. This is usually achieved by switching attention back and forth rapidly, jumping from the one screen to the other, quickly doing the control moves necessary.

A good multitasker should be able to macro constantly throughout the game, while being able to micro units and being alert to any attack/harass they encounter.

An example could be performing a frontal attack, drop-harassing a mineral line and keeping up your macro production at the same time.