The BIOS is the motherboard's firmware. Firmware is the software that allows the motherboard to communicate with all the hardware on it, i.e RAM, CPU, PCIe slots, etc.
This warning means that this motherboard MAY POSSIBLY come with an outdated BIOS firmware version that does not provide support for the 4790 intel CPU. I emphasize 'may possibly' because since this an older board it may come updated already. If possible confirm with the seller if the BIOS has been updated to support the 4790.