In computer terminology, the definition of multitasking is; a method where multiple tasks, also known as processes, share common processing resources such as a CPU. Multitasking achieves parallelisms in executing multiple tasks at a time. Multitask execution happens by rapid switching among several tasks. Thus it appears to be all the tasks are being executed simultaneously.
So CPU executes a process for a while and then process switching is done and another process starts executing and this process goes on, thus making progress of each process in small fragment. Also every switching is done the first process execution state is saved and 2nd process state gets loaded into memory for execution to start. In This fashion time is wasted (although tiny amount) for switching between every two processes.