"This line shows the single attack the creature makes with an attack action. In most cases, this is also the attack the creature uses when making an attack of opportunity as well."
http://www.d20srd.org/srd/monsters/intro.htm#attack
Now "10 bites" is the single attack that the hydra makes. It can make "10 bites" whether it moves or not; this is an attack action. Also, apparently, it can make "10 bites" whenever it makes an Attack of Opportunity. Now, the hydra might be an exception to this rule, but I'm not aware of this. Perhaps the Hydra has been specifically discussued in a FAQ or errata document.
By the way, I think that this rule, if it's accurate, is crazy. I have heard it quoted in many places, though, and read many arguments about how dangerous the Hydra is.