HSYNC and VSYNC signals in VGA pins (FPGA programming).

I do not understand what the use of HSYNC and VSYNC is when connecting an FPGA board through VGA to a display. There are rgb signals, and hsync, vsync.

I've read numerous resources on web but I'd be grateful if someone can explain the need of these 2 signals and how the logic of creating them on FPGA normally is.

Thanks in advance.