Sunny 16 is a rule to get correct exposure. if you use a fully manual camera (film) then this rule is a must know. Instead of repeating what others on the net has already explained let me just copy and paste it here. This is taken from http://www.slrlounge.com/photography-essentials-the-sunny-16-rule , all credits goes to them. This is a good read.
WHAT IS THE SUNNY 16 RULE?
WHAT ABOUT OTHER APERTURE VALUES?
|Change in Aperture||Stop Difference||Change in Shutter||Stop Difference|
|f/11||+1 stop||1/400 @ ISO200||-1 stop|
|f/8||+2 stops||1/800 @ISO200||-2 stops|
|f/5.6||+3 stops||1/1600 @ ISO200||-3 stops|
|f/4||+4 stops||1/3200 @ISO200||-4 stops|
|f/2.8||+5 stops||1/6400 @ISO200||-5 stops|
HOW IS THE SUNNY 16 RULE USEFUL?
i also found exposure mat, go here http://expomat.tripod.com/ It is based on Sunny 16 rule.
And also there's an iphone app
and this one is for free