A multitasker is someone who does several things at the same time — the person, not the action. In everyday English it almost always means a human who juggles more than one job at once, and people often say it with a little praise: “She’s a great multitasker.” Though actually, it can also be a polite no: “I’m not a multitasker” means “please give me one thing at a time.” The action is to multitask; the person is the multitasker. Older computer talk could use it of a machine, but that use has mostly faded.