1錛塒repreg

What is the purpose to use Prepreg?
Prepreg is used to stick the core layers together and to stick core and copper foil together.

2錛?What is Core?

The core-layers being FR4 with copper traces above and below.
The layer stack (multiple of: cores, prepreg & copper foil) is pressed together at temperature to the required board finish thickness.
It is clear from the above diagram that for 6-layer PCB stack up, we need 2-cores, 2-copper foils & prepregs at 3 places (each place can have more than one layer to achieve the required thickness).