That means for example : if the campain contains layouts 1, 2 and 3, displaying layout1 for 10 sec., layout2 for 15 sec., layout3 for 5 sec., then back to beginning displaying layout1 for 10 sec., etc.
So if you create a campaign, assign your 3 layouts to it and then schedule it to display on your device for lets say an hour it will loop exactly as you described ie - layout 1 layout 2 layout 3 (the same order as in the campaign) then back to layout 1 and so on till (in this example) 1 hours will elapse.
So with 900s collection interval, when there was some connection issue everything was delayed for next 15min, with 30s collection interval even if there is something wrong (ie one connection would fail) then next attempt is only 30s away so it might be better overall.