It's the error code of the shell command that make was trying to execute. For example, if the Makefile has a command
and directory /fu/bar/lib already exists, then mkdir will return an exit status of 1. make will output make and possibly a message.
make itself only has three exit status codes:
0 = make was successful.
1 = if you use the `-q' flag and make determines that some target is not already up to date.
2 = make encounters any errors. It will print messages describing the particular errors.