what is SLAM in robotics?

The problem it solves is circular: to make a map you need to know where you are, and to know where you are you need a map. SLAM solves both at once, dozens of times per second, which is why a modern AMR is installed by walking it through the warehouse just once.

In practice there are two families: laser SLAM, the industrial standard used by robots like the Locus Origin, and visual SLAM with cameras, cheaper but more light-sensitive. What that difference means when buying is explained in AMR vs. AGV.