A reserve army of labour is like having a group of people who are not currently working, but who are available and willing to work if they are needed. Think of it like having a bunch of superheroes waiting in the wings, ready to step in if the regular heroes are not available.
Sometimes companies have times when they need more workers, and other times when they need fewer workers. So they might hire some people when they need them, but then let them go when they're no longer needed. The people who are let go become part of the reserve army of labour.
One important thing to remember is that people in the reserve army of labour often have a harder time finding work and may not get paid as much as people who have permanent jobs. It's kind of like getting picked last in a game of dodgeball, except instead of being picked last, you might not get picked at all.
Governments and economists keep an eye on the reserve army of labour because it can affect things like wages, unemployment rates, and the overall health of the economy. So even though it might seem like a bunch of people just waiting around to work, it's actually a really important part of how our economy works.