FTW list files recursively C

I am new to File Tree Walk. I am having troubles searching for files recursively. I tried to look up some documentations but the wordings are quite confusing. This is what I have (not the implementation), but I don't really understand these arguments:

int ftw(const char *dirpath, int (*fn) (const char *fpath, const struct stat *sb, int typeflag), int nopenfd);

int nftw(const char *dirpath, int (*fn) (const char *fpath, const struct stat *sb, int typeflag, struct FTW *ftwbuf), int nopenfd, int flags);