The creature doesn’t get its normal complement of actions on its turn; it can take either a standard, a move, or a minor action. The creature can still take free actions.
The creature can’t take immediate actions or opportunity actions.
The creature grants combat advantage.
The creature can’t flank.
The dazed condition is a common way of representing a general state of stupor. Dazed creatures don’t act as quickly as normal or respond to danger as readily. A creature might be dazed because it took a hard blow to the head, a spell assaulted its mind, or a prayer called forth a clap of thunder around it.
Published in Player's Handbook, page(s) 277, Player's Handbook 3, page(s) 220, Rules Compendium, page(s) 229, Monster Vault: Threats to the Nentir Vale, page(s) 122.