What does * * * * * (five asterisks) in a cron file mean?

Solution 1:

Every minute of every day of every week of every month, that command runs.

man 5 crontab has the documentation of this. If you just type man crontab, you get the documentation for the crontab command. What you want is section 5 of the manual pages which covers system configuration files including the /etc/crontab file. For future reference, the sections are described in man man:

   1   Executable programs or shell commands
   2   System calls (functions provided by the kernel)
   3   Library calls (functions within program libraries)
   4   Special files (usually found in /dev)
   5   File formats and conventions eg /etc/passwd
   6   Games
   7   Miscellaneous  (including  macro  packages and conven‐
       tions), e.g. man(7), groff(7)
   8   System administration commands (usually only for root)
   9   Kernel routines [Non standard]

Solution 2:

* = always. It is a wildcard for every part of the cron schedule expression.

So * * * * * means every minute of every hour of every day of every month and every day of the week.

 * * * * *  command to execute
 ┬ ┬ ┬ ┬ ┬
 │ │ │ │ │
 │ │ │ │ │
 │ │ │ │ └───── day of week (0 - 7) (0 to 6 are Sunday to Saturday, or use names; 7 is Sunday, the same as 0)
 │ │ │ └────────── month (1 - 12)
 │ │ └─────────────── day of month (1 - 31)
 │ └──────────────────── hour (0 - 23)
 └───────────────────────── min (0 - 59)

The nice drawing above is provided by wikipedia

An other example:

0 * * * * -this means the cron will run always when the minutes are 0 (so hourly)
0 1 * * * - this means the cron will run always at 1 o'clock.
* 1 * * * - this means the cron will run each minute when the hour is 1. So 1:00, 1:01, ...1:59.

---

Solution 3:

First star = Minutes: 0-59
Second star = Hours: 0-23
Third star = Day of Month: 0 - 31
Fourth star = Month: 0 - 12
Fifth star = Day of Week: 0 - 6 (0 means sunday)

Say you want to run something every 1st of every month.

0 0 1 * * something.sh