Detailing a little more:
'make' not being a directive of the shell, the shell assumes it must be a separate program and searches in the directories listed in the PATH variable (type 'echo $PATH' to print its content) for a file named "make". It finds it in /usr/bin and runs it (passing to it the space-separated arguments following "make", e.g., "install"). Then, 'make' looks for a makefile, etc.
