Glue-like dielectric between copper layers; the style number sets thickness (2116 ~0.1 mm, 7628 ~0.2 mm).
How it looks
Drawing from our real stackup (board file setup)
CORE3588 has two prepreg layers, one under the top copper and one above the bottom copper, each 0.105 mm of style 2116.
Drawing: prepreg is buried inside the board, so it cannot be photographed from outside
What a prepreg sheet is: woven glass cloth soaked in epoxy resin and partly cured, so it is still tacky.
How it works
Where it fits
Prepreg is one of the layers of the stackup; the other insulator is the core, together the substrate. Its thickness and material feed the impedance of every track on the outer layers.