Multitasking
Multitasking is trying to do two or more things at once, which usually makes each one harder.
Think of it this way
Two pots sit on the stove at the same time. Each pot needs a stir, but you only have one ladle. Stir one pot and the other boils over. Neither pot gets full care when you split your watch between both.
Explaining multitasking by grade level
Try to sing a song while you draw a picture. Both get harder when you do them at once. Your brain can only focus on one thing well at a time. When you split your focus, you slow down and make more mistakes.