Right now it takes 15 blocks before transactions in a past block are irreversible (transactions inside can't be canceled even if the computers processing hive transactions have network problems). This is a proposed change to how these computers process transactions, which would allow transactions to generally be irreversible after just one block. In other words, right now it takes at least 45 seconds to be sure a hive transaction has definitively been accepted. With the change, this will normally happen in 3 seconds or less (most commonly in less than 1 second).