It basically says that there is a ferry from Split to Supetar (Brac) every 90 mins or so during the peak season starting 05:15 until 21:00, with another one at midnight. Journey time is around 45-50 mins.
Its busy all the time. But as it has been said before: Weekends (Fri-Sun are the worst). You should get there at least half an hour early if you are taking a car but an hour would be better.
There's also a private ferry company from Split to Supetar is which is called Split Tours (formerly SEM). Their web site seems to allow reservations:
http://www.splittours.hr