You can not actually multitask
What research suggests is that we are actually switching back and forth between the demands of the tasks rather than actually simultaneously doing all the tasks. If the tasks require the concurrent sensory input, processing or motor functions, we have to divide our abilities up or split our attention. An instance of two tasks that require the same sensory inputs is when we try to imagine seeing something in the head and seeing what is in front of us.
In the source article (below), it also indicates that multitasking reduces ability to organize and store information, and people who engage in multitasking actually becomes more prone to distractions.