The layer stack-up defines the number and thickness of the layers that are sandwiched together to make a PCB. PCBs are made from ‘cores’ and ‘prepreg’. Cores are solid fibreglass boards with copper on both sides, while prepreg is a semi-set resin cloth. PCBs have to alternate between a layer composing of a core and then a layer of prepreg. These are sandwiched together, pressed and heated to make the final PCB. Below is an example of a stack-up for a four-layer, 0.8mm thick PCB.
There is some great PCB layer stack design software at https://isodesign.isola-group.com/isostack/.