Most of the time I was ordering PCBs made with KiCAD at places like pcbway, dirtypcbs, and it all went well: you just type in your requirements in their online form, wait several weeks and you have your PCBs.
But online PCB houses are not everything, right? There are some places that dont have fancy online forms for all the needs. These guys rely on your CADs fabrication output. One type of fabrication output that KiCAD is missing is PCB stack-up.
Im not an expert in PCB manufacturing, and good stack-up
PCB house should know how many layers are there in your PCB, what thicknesses they have and how layers are ordered.
The number of layers includes both metal layers and dielectric layers , which can be of two types: core and pre-preg. All of the layers have standard sizes. As far as I know, to achieve required thickness, the pre-preg layers can be glued together to form thicker layer, but this is usually not your headache, unless you are working with higher frequencies.
Stack-up editor lets you open your kicad_pcb file, set layer thicknesses, and plot the diagram to pdf file, which can then be sent to factory.