There are multiple sections in the manpages. Two of them are:
2 Unix and C system calls 3 C Library routines for C programs
For example there is getmntinfo(3) and getfsstat(2), both look like they do the same thing. When should one use which and what is the difference?
System calls are operating system functions, like on UNIX, the malloc() function is built on top of the sbrk() system call (for resizing process memory space).
Libraries are just application code that's not part of the operating system and will often be available on more than one OS. They're basically the same as function calls within your own program.
The line can be a little blurry but just view system calls as kernel-level functionality.