Unlock 1Password Keychain
1Password can only store or access your confidential information when the 1Password keychain is unlocked. When locked, you will be asked to provide your Master Password before it can be used.
In a Web Browser the unlock screen will look similar to this:

The Keychain Locked screen in the main 1Password application looks similar to the above Browser window.
To unlock and use the keychain, enter your Master Password.
What is the 1Password Keychain and Master Password?
1Password stores all of your confidential information into a keychain that is encrypted using your Master Password to keep all your information safe. The password used to encrypt the keychain is the one provided to 1Password upon initial setup:

When you are prompted to unlock the keychain, this password is what is needed to open the keychain before it can be use.
Why is the 1Password Keychain Locked?
By default the keychain is locked automatically if the keychain is not used for 60 minutes or when your computer sleeps. You can configure these settings in the 1Password preferences panel as discussed below.
The keychain can also be locked manually from the 1Password application Lock Keychain toolbar or the Keychain Access application.
Changing the Keychain Locking Policy
By default the 1Password keychain is configured to lock automatically when your Mac sleeps, as well as after 60 minutes of inactivity.
Having your keychain lock when sleeping and after a period of inactivity are usually great security features, but in some environments having the keychain lock so often is more of a pain than a feature.
Thankfully the keychain locking preferences are completely configurable. You can change keychain settings using 1Password > Preferences > Keychain menu.
Unlock Automatically Using Login Keychain
If desired, you can store your password into the default login keychain to enable automatic login. To do this, enable the “Remember password in login keychain” checkbox on the Unlock Screen.
If enabled, you effectively delegate all the security of 1Password to the strength of your Login Keychain settings. It is therefore important that the login keychain is properly secured with a strong password and set to lock on sleep.
Forgotten Password
If you have forgotten your password, or cannot get any of your passwords to work, see Forgotten Password