How does BIOS communicate errors?

The biOS reports errors on the start-up screen. Simply white on black. It reports any failures to start up correctly, for example if an operating system is not found, or a vital component is not functioning.
If the screen memory or basic video cannot be accessed, the error can be signalled by a series of beeps from the onboard sounder. The exact meanings of the beeps, will have to be looked up in the motherboard instructions.