How do I unlock portrait mode in iPhone?
This is the lock rotation icon. You will notice that when you turn your iPhone (say, in Safari) it will stay in portrait.

To disable it:

On iOS 7+ : 


Open the Control Centre by sliding your finger from the bottom of the screen to the top
Tap the button a the top right (here circled in red)





iOS 6 


Double tap on home button
Then slide the multitask bar to the left (where the Sound controls are) 
And on the left you'll find the button to unlock this.