And to add to that a little more detail...
In a normal MTT, tables get broken up to keep things balanced. Not so in a shootout. In a double shootout (considering 9 players per table), it would typically start with 81 entries (9 tables of 9 players). Each table plays down to one person (sort of like a single table tourney). Once each of the nine tables has a table winner, those nine table winners are put at one table and they play it out to the end. That's a double shootout.
In a triple shootout all you do is start with 729 entries (this is a WCOOP event on PS btw). That is 81 tables of 9 players. Each table plays down to one table winner, leaving 81 table winners when done. From there the rest is like a the double shootout above. 81 players down to nine table winners to final table and done.