Complex sentence: Pluggable authentication modules or PAM are a mechanism to integrate multiple low-level authentication schemes into a high-level application programming interface (API), which allows programs that rely on authentication to be written independently of the underlying authentication scheme.
Split into simpler sentences: Pluggable authentication modules or PAM are a mechanism to integrate multiple low-level authentication schemes into a high-level application programming interface (API). They allow programs that rely on authentication to be written independently of the underlying authentication scheme.