Using PIN code or fingerprint
You can go to More > PIN Code and turn the lock on to create a new PIN code.
If you forget your code see this tutorial.
If you lock your Daylio with a PIN lock and your phone has activated fingerprint authentification, Daylio will automatically use the fingerprint for unlocking the app.
System app lock or 3rd party solution
You can also use system or 3rd party solutions to lock Android apps. For more information see this article https://www.lifewire.com/lock-apps-android-4583099 or google "lock android app".