require is a built-in function in `C source code'.

(require FEATURE &optional FILENAME NOERROR)

If feature FEATURE is not loaded, load it from FILENAME.
If FEATURE is not a member of the list `features', then the feature
is not loaded; so load the file FILENAME.
If FILENAME is omitted, the printname of FEATURE is used as the file name,
and `load' will try to load this name appended with the suffix `.elc' or
`.el', in that order. The name without appended suffix will not be used.
See `get-load-suffixes' for the complete list of suffixes.
If the optional third argument NOERROR is non-nil,
then return nil if the file is not found instead of signaling an error.
Normally the return value is FEATURE.
The normal messages at start and end of loading FILENAME are suppressed.