...used in Unix to guess the effectibe datatype of a file. It's some bytes at the beinning of the file that contain a signature of the file type.
Unix and Linux use them to know which executable format is given to execute. If you experience this error, either the file is corrupted, it is truncated, you have filesystem errors (cross-linked clusters or sectors), ...
Or you want to execute a program that has not been built for your OS, and Unix/Linux refuses to launch it!