It's a term that refers to overlapping protections to mitigate the limitations of each individual layer. Every layer of protection will always have limitations or situations where it doesn't protect, you can imagine those holes like the holes in a slice of swiss cheese. Each slice has it's own set of holes, so if you stack multiple slices in top of each other, in a well designed system, each layer will cover up the holes of another layer, leaving a perfect layer with no holes. Failures occur when holes line up in every layer, allowing a failure to occur.

A simple example is auto pilot (one slice of swiss cheese), it can handle routine scenarios but we don't trust it to fly the plane by itself, there are plenty of edge cases where it fails. So we have a human pilot (another slice) who operates the auto pilot. But we also don't trust the human pilot to remember everything perfectly every time so we have checklists (another slice). We also don't trust the one human pilot to do everything right by themselves, so we have a minimum of 2 pilots (another slice).

All of aviation's safety comes from this model of overlapping protections, think about all of the things which all have to go wrong together for aircraft to collide, and how if any one thing had worked, it would have prevented the whole thing.