An action represents any task that takes a relatively significant amount of effort. A character can take any number of actions in a round, so long as the total time taken by them adds up to six seconds. In one second, a character may (for example) move or draw a weapon. In four, he could use a combo or cast a spell. This means that in a full round, he could move six times, use two combos, cast a spell and move three times, or any combination thereof. Action time is sometimes abbreviated, so a 4-second action could be written as a “4SA.”
Unless otherwise specified, actions that take longer than 1 second must use concurrent seconds. So a character couldn't spend 3 seconds casting a spell and then an hour later use the final second to trigger it, all 4 seconds must be done successively. This doesn't mean that the whole activity must be done in one round, though. A character can, for example, spend the last 3 seconds of one round casting a spell and finish it on the first second of the next round.