There are loads of header files available. Header files allow access to declarations and macro definitions.
time.h provides functions relating to time e.g. start a timer
System file headers (already available) are accessed by #include <name.h>
Can make your own header files and access by #include 'name.h'. Simply make a file with .h extension and have own functions etc inside and include it. Then in main script, call the function names, and it will go to relevant .h file