This article is from the Magic: The Gathering Rules FAQ, by Patrik Linell (pls@claymore.nu) with numerous contributions by others.
Phasing is a 'disadvantage' ability, which essentially means you
only have your creature (or whatever it is that has the ability) on
alternate turns.
At the start of your turn, just before everything untaps, all your stuff
with Phasing phases out. (note that your opponent's stuff stays where it
is- it's just the active player who does this.)
Most players turn their cards face down to show they're phased out.
While something is phased out, it's essentially out of the game- it can't
be targetted, can't attack, can't use its abilites and so on.
At the same time, all your stuff that's already phased out phases back
in. They'll still have all the counters and enchantments that were on them
when they phased out, and so on. Creatures that phase in have Haste until
their controller's next turn begins.
So, for example- You play a Breezekeeper, a 4/4 phasing creature. You
can't attack with it this turn because it just came into play. It can block
in your opponent's turn.
On your next turn, it phases out. So you can't attack with it this turn. It
can't block on your opponent's turn.
On your turn after that, it phases back in, so you can finally attack with
it, or block on your opponent's turn.
On your turn after that, it phases back out... you get the picture?
Phasing details:
If a creature phases out tapped, it will phase back in tapped. Since
this normally happens at the start of the turn, just before you untap
everything anyway, it doesn't usually matter.
When a creature phases out, abilities that trigger "when ~ leaves play"
will do so. When it phases in, abilities that trigger "when ~ comes into
play" will _not_ do so. Other than reasons of game balance- this would be
too powerful with Bone Shredders, for example- there's no real explanation
for this behaviour. Just stick Vanishing on your Thalakos Seer and enjoy it.
 
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