RAR archive of 2.57 MB for the Apple logic board 820-3437; the 43 prefix appears to be an index number. Apple's 820-xxxx numbers are printed on the board and are the reliable way to match documentation, since one model name can cover several boards. The file name does not say whether it holds a schematic, a boardview or firmware, so check after extracting. Such material helps trace power rails, find shorted components, locate test points and work through the power-up sequence on a MacBook 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 @SCHEMATICWORLD. 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.