Serverless is a relatively new marketing term, old by tech world standards, whereby you "don't need infrastructure". Your code gets executed into sort of virtual machines "small enough" to boot up on demand and execute your specific code. You don't need to manage dependencies or os configs. You define an "entry point" within your chosen programing language and the vm executes it on demand. The vm stays active for a set time. Beyond that time a new request requires a "cold start" - this can be slow if the OS' boot time is slow.

Also this is meant to make things easier and cheaper. But they are anything but easier and anything but cheaper compared to non serverless. It also leads to captive customers. But it drives a whole industry of training, consulting and selling AWS services.