Recent testing revealed that Arch Linux, Pop!_OS, and even Nobara Linux, which is maintained by a single developer, all outstripped Windows for the performance crown on Windows-native games. The testing was run at the high-end of quality settings, and Valve's Proton was used to run Windows games on Linux.
I interpret it as the time taken to render a frame. Unlike FPS which is basically a moving average (or rather 1 divided by the average frame time), frame time is a single data point. Collecting frame times allows you to do things like compute the median or, in this case, the lowest 1% of the frame times. That can give you a better idea of how smooth performance appears to the player, and what the worst-case performance is like.