It appears that "Common Tasks" refer to tasks that everyone has; i.e. tasks that are common to everyone. From the wiki
Common tasks are given to every single Crewmate. For example, if one crewmate has to Swipe Card, every crewmate must swipe a card. Conversely, if one Crewmate does not have Swipe Card, this means nobody has a Swipe Card (especially not Impostors!) Common tasks are an easy way to rat out Impostors, for if you see someone doing a common task that you don't have, you can pretty much confirm that they're An Impostor.
So for example, if you see someone hanging around the Electrical panels, or they claim they were "fixing wiring", and you don't have that task, then they are definitely an Imposter! Imposters do have a list of tasks to fake, but it's unlikely they'll pay attention to that, because they'll be more concerned with figuring out how to "off" the rest of the crew.
The list of common tasks are:
- Enter ID Code (MIRA HQ)
- Fix Wiring (all maps)
- Insert Keys (Polus)
- Scan Boarding Pass (Polus)
- Swipe Card (The Skeld & Polus)
Additionally, for Imposters the only things that "light up" (like the appropriate panels for certain tasks do for normal crewmates) are the potential targets to murder, tasks to fix sabotage, and vents (as well as general panels such as the Map and the Security Cameras). Panels that relate to tasks do not light up, so they might accidentally mistake a wiring panel for the power diversion panel, being another good giveaway.