Voltage Regulator Modules. They are what regulate the power that feeds the CPU. The higher end Z370s that are made for overclocking use a 10 phase VRM, the board you are looking at has 6 phases. Less VRMs means they have to work harder which creates more heat. CPU overclocking requires more power so the fewer VRMs will have to work even harder which makes problem like VRM overheating and/or stability issues with the voltage which will cause system instability/crashes.