The crond(8) manpage says to reference the crontab(5) manpage for an
explanation of the special syntax of the files under /etc/cron.d.
However, that manpage says nothing about them whatsoever that I can find.
They should be fully explained, primarily the fact that they require a
userid to run under.
The crontab(5) manpage explains crontab file syntax.
To edit system crontab files, you must be user root.
To edit per-user crontab files, use the crontab(1)