Working memory is the small, busy space where you keep things in mind right now. It is smaller than most people expect. Watch the squares light up, then tap them back in order. Each round adds one.
Most adults top out at around four items held at once. The famous "seven plus or minus two" has been revised down. Capacity is limited, it varies from person to person, and it is closely tied to attention and learning.
New material has to pass through this bottleneck. Pile on too much at once and it simply spills. That is why chunking, worked examples, visuals, and building on what students already know all help: they lighten the load on working memory.