Select Lock Collection in the Collection menu to lock the collection with the password that you previously set with the Set Password command. When the collection is locked its tab will be yellow and the collection will appear to be empty. The AceText Editor will display a box where you can enter the password to unlock the collection.
In the Files Preferences you can enable various options to automatically lock collections in certain situations. Use these options to make sure you don't accidentally leave sensitive collections unlocked.
There is no toolbar button for the Lock Collection command. If you've hidden the main menu, you can access the command by right-clicking on a collection's tab.