A Multitasking role is a role that allows its owner to perform more than one active night action in the same night; in other words, if the player has multiple night choices that refer to different actions, they are not mutually exclusive and can be performed at the same time. Typically, any number of different actions could be used at once. For example, a Multitasking Mafia Watcher could use both their "watch" action and their factional kill in the same night, if they chose to. There is no restriction on targets (other than the usual restrictions against self-targeting), e.g. the watch and kill could be placed on the same target or on different targets.
Multitasking does not allow a player to use multiple copies of the same action in the same night; the aforementioned Multitasking Mafia Watcher would not be able to watch two different targets in the same night. Multitasking also does not allow a player to exceed a limit on how often a single action can be performed; thus, for example, it would not be possible to perform the factional kill at the same time as another member of the faction was performing the factional kill, because of the restriction that the Mafia can only collectively perform their factional kill once per night.
In setups where players can normally only perform one action per night, the fact that you're multitasking will become obvious if anyone tracks you. Because, outside role madness, it's far more common for Mafia to have a choice of types of night action than it is for the town (as the Mafia's factional kill forms a "natural" second action alongside a power role, whereas it's rare for townies to have two different power roles), this modifier is most commonly seen in anti-town hands. As such, being caught multitasking may be hard to explain (a claim of Multitasking Jack of All Trades may not be believable). On the other hand, if one of your actions is performing the factional kill, then being tracked to the kill is going to be fairly hard to explain away anyway, so it may make more sense for the scum to "put all their eggs in one basket" and simply hope to dodge any tracking attempts rather than split up their actions between different members of their team. Perhaps a good rule of thumb is for the scum to perform their kill using a member that they're willing to sacrifice, or failing that, the member least likely to be tracked or roleblocked, whether or not they happen to have a multitaskable second action on the side. (Of course, if a player has multiple possible actions and is not capable of multitasking, sending them to perform the kill would be something of a desperation move.)
Note that despite being a modifier associated with scum, it is probably a bad idea to claim this at the start of Day 1 (like you would with Miller) if you get it as town, as you'll just make yourself an obvious kill target. Simply hope you can explain away your actions easily if someone discovers you have it.