Here is how to lock apps on iPhone without downloading any additional stuff. If you are using Android, use this app to lock apps for android.
Lock Apps on iOS with Guided Access
There is one built-in feature that you can use to lock apps & stop others from snooping around, but it isn’t the greatest thing in the world. In the Accessibility settings, you’ll find Guided Access, which will allow you to keep your iPhone in a single app and control which features are available to it.
Once Guided Access is enabled, use the triple-click home button feature to bring it up. Draw across the screen to block certain areas from being accessed, then tap Start. Once Guided Access is up and running, no one will be able to touch the parts of the screen you disabled unless they know your passcode.
This nifty trick could prove useful to stop others from reading all of your text messages—as you can see in the picture above, I blocked all text messages (in the grey) but the one i wanted to show.
When you want Guided Access gone, just triple click the home button and enter your passcode to get rid of it.
We previously covered this feature to show how to keep friends from snooping through your pictures, so if you want more information about it, head over to that article for more info.
If this is too messy or cumbersome for you to lock apps, there are applications out there that you can use to lock apps. For the list of applications available (android), click here.
Originally posted by Nelson Anguilar on WonderHowTo.