RAR archive of 1.62 MB for Apple board 820-3208, revision A; the name gives no model number or description of contents. Check the 820 number and revision letter printed on your logic board before relying on the material. Unpack it with WinRAR or 7-Zip to find the schematic and any boardview file. A technician uses these to trace rails, follow signal names between chips, check component values and track down shorts on main supply lines of a dead or unstable board.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @Mac_Shematic_Santale. Enginerys does not host this file.
How to open a .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.