A Phased tournament is one in which some number of levels at the beginning of the tournament are played non-concurrently from the end of the tournament, for different groups of players.
Players can chose from different starting phases that begin at different times. Each starting phase will play for the same amount of time, and then all remaining players will later combine in a single, larger tournament. Phased tournaments allow the schedule flexibility normally associated with smaller-field tournaments, while still enjoying the large prize pools associated with larger-field tournaments.
Chip counts at the end of Phase 1 will be carried over into the next round.
For example, a tournament might have Phase 1 on Friday at 12:00, another Phase 1 on Saturday at 12:00, and then conclude with a Phase 2 tournament on Sunday at 12:00. Entrants would play for a specified number of levels (or until elimination) on Friday and/or Saturday, and all players that survive would combine to finish the tournament at 12:00 on Sunday.
When you enter a Phased tournament you must ensure that you will be available to play in the future phases, as you cannot unregister from Phase 2 (or beyond) in Phased tournaments.
Phased tournaments allow for multiple entries into the first phases. For example, if you play Phase 1 and are eliminated, you may enter another Phase 1 and start again at the beginning. You cannot qualify for the next round more than once, so if you survive Phase 1, you will then be able to enter another Phase 1 leading to the same Phase 2 but only the highest stack you finish with will be taken to phase 2. Note that if you survive Phase 1 with even one chip, you will still advance to Phase 2. If you qualify more than once, the lower of your stacks will be forfeited.
Most of the high-profile tournaments seen on TV are essentially phased tournaments, with the entry phases usually referred to as Day 1A, Day 1B, Day 1C, etc.