HI_START is a marker that is used in conjunction with HI_END. It is used to define high resolution textures that already exist and scale them according to the original texture. This is handy if you wish to create a wad which replaces the original game's textures with hi-res ones, no matter what size, without the use of typing up a TEXTURES lump.
Note that if you try to replace a non-existent texture, the image will be used unscaled as if it had been placed between the TX_START and TX_END markers (the same namespace is used). It is better practice, however, to put such textures between the proper markers.
If you are using ZIPs as WAD replacement, the equivalent directory is "Hires".