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Every minute, a cron job is checked whether it should run or not.
<pre>*/10 * * * *</pre>
is equivilant of
<pre>0,10,20,30,40,50 * * * *</pre>
and runs every hour at the minutes 0,10,20,30,40,50.
If for some reason (connection congestion, server crash, etc.) that Ultimate Cron cannot launch the job, e.g. 10:30, then normally, the job won't attempted to be launched again until 10:40.
The <em>catch up</em> compensates for this, allowing the job to be launched anyway until a certain time. The <em>Catch up</em> value is in seconds, so if it is set to 300, the job will launch at 10:31-35 if it did not launch at 10:30
