A BIOS dump is a full image of the board's SPI flash chip. It is written back with an SPI programmer when the original firmware is corrupted, after a chip replacement, or to recover from a failed update. Read and save the original chip first: serial number, DMI data and the Intel ME / AMD PSP state differ between boards.

Tools to open these files

Format	What it is	Open with
.pdf	PDF schematic	any PDF reader; Sumatra PDF and Foxit handle large schematics well
.rar	RAR archive	7-Zip or WinRAR
.bin	BIOS image inside the archive (usually .bin)	a CH341A, XGecu T48 / TL866II Plus or RT809H programmer