Poker sequence refers to the ordering of the cards. For example, five consecutive cards in sequential poker rank order make a straight in poker. Thus, if you hold consecutive ranks, i.e., 6,7,8,9, 10, you can refer to them as being in sequential order.
Basically, the rule states that you must make ten times the raise size from the total pot when set mining, twenty-times the raise size when calling with suited connectors, and thirty times the raise size from gapped suited-connectors.