man 5 crontab show that 'step values' can be used:
Step values can be used in conjunction with ranges. Following a range with
'/<number>' specifies skips of the number's value through the range.
For example, '0-23/2' can be used in the hours field to specify command execution
every other hour (the alternative in the V7 standard is
'0,2,4,6,8,10,12,14,16,18,20,22'). Steps are also permitted after an asterisk, so if
you want to say 'every two hours', just use '*/2'.
So in your case this is "run every minute". Most crons will use a granularity of 1 minute,
* is exactly the same (and is actually more "portable" as not all cron servers support