Imagine you are watching a game of chess. You keep track of the time since the players started on your wristwatch. This is the runtime.

The chess players have a clock on the table which only ticks when its the respective players time to make a move. This is the CPU time.

If a player is playing several games concurrently, that players clock is ticking on all the tables whenever its that players time to make a move. The time spent by a player playing several matches concurrently may rapidly increase beyond your wristwatch.