0 /2 * * — every two hours

Answer

0 */2 * * * runs at minute 0 of every even hour: 00:00, 02:00, 04:00 … 22:00, twelve times a day.

At minute 0 past every 2nd hour (00:00, 02:00, 04:00, 06:00, … 22:00).

0
Minute (0-59)
At minute 0 (on the hour)
*/2
Hour (0-23)
Every 2nd hour starting at 00:00: 00:00, 02:00, 04:00, 06:00, … 22:00
*
Day of month (1-31)
Every day (no restriction)
*
Month (1-12 or JAN-DEC)
Every month
*
Day of week (0-7 or SUN-SAT (0 and 7 = Sunday))
Every day (no restriction)

12 runs a day on average84 a week364 of the next 364 days have a run

Hours of the week (darker = more runs)

The next 52 weeks, one square per day

What to know

*/2 in the hour field selects 0, 2, 4 … 22. For the odd hours, start the step at 1 with a range: 0 1-23/2 * * * runs at 01:00, 03:00 … 23:00.

0 1-23/2 * * *
every 2 hours on odd hoursAt minute 0 past 01:00, 03:00, 05:00, 07:00, 09:00, 11:00, 13:00, 15:00, 17:00, 19:00, 21:00 and 23:00.
30 */2 * * *
every 2 hours at half pastAt minute 30 past every 2nd hour (00:00, 02:00, 04:00, 06:00, … 22:00).

*/2 * * * * (the step in the minute field) is every 2 minutes, a very different load.
Hours are in the cron daemon's time zone, usually the server's. On a UTC server 00:00 is not your midnight.