Your senses allow you to notice fine details and alert you to danger. Perception covers all five senses, including sight, hearing, touch, taste, and smell.
Perception has a number of uses, the most common of which is an opposed check versus an opponent's Stealth check to notice the opponent and avoid being surprised. If you are successful, you notice the opponent and can react accordingly. If you fail, your opponent can take a variety of actions, including sneaking past you and attacking you.
Perception is also used to notice fine details in the environment. The DC to notice such details varies depending upon distance, the environment, and how noticeable the detail is. The following table gives a number of guidelines.
1 Favorable and unfavorable conditions depend upon the sense being used to make the check. For example, bright light might increase the DC of checks involving sight, while torchlight or moonlight might give a penalty. Background noise might reduce a DC involving hearing, while competing odors might penalize any DC involving scent.
2 As for unfavorable conditions, but more extreme. For example, candlelight for DCs involving sight, a roaring dragon for DCs involving hearing, and an overpowering stench covering the area for DCs involving scent.
Most Perception checks are reactive, made in response to observable stimulus. Intentionally searching for stimulus is a move action.
Retry? Yes. You can try to sense something you missed the first time, so long as the stimulus is still present.