It's similar to Linux cron, but there's a ? involved.
They thought it would be too confusing to put * on both Day of Month (column 3) and Day of Week (column 5). So, one can be * but the other must be ?.
# e.g.
cron(0/5 * * * ? *)
^ every five minutes
^ every hour
^ every day of the month
^ every month
^ Day of week is unknown
^ Every year
# e.g. once an hour, but only at 12-6 PM UTC
cron(0 8-17 * * ? *)
# e.g. once an hour, but only at 12-6 PM UTC and only on Tuesdays
cron(0 8-17 * * TUE *)
# e.g. once an hour, but only at 12-6 PM UTC and only on Work days
cron(0 8-17 * * MON-FRI *)
Other examples:
https://docs.aws.amazon.com/AmazonCloudWatch/latest/events/ScheduledEvents.html