This is a specific revision of the first disc of the original Metal Gear Solid for the PlayStation 1 (PSX/PS1), released in the USA. The "(Rev 1)" indicates it’s a later pressing that includes bug fixes and minor adjustments compared to the original "Rev 0" release.
Here’s a useful write-up regarding the specific ROM/ISO file: Metal Gear Solid (USA) (Disc 1) (Rev 1) – A Preservation & Emulation Guide Download Metal Gear Solid -USA- -Disc 1- -Rev 1-