Let's face it we are very dependant on our phones these days. Often we store personal photos, videos, and documents that we would never want anyone else to have access to (just imagine if you lost your phone today, is there anything on it you don't want anyone to see?).
The topic of keeping your phone's files private is often referred to in different ways: hiding files, encrypting files, password protecting files, locking files, etc. And they are not all equal in terms of how safe your files are. For example, hiding files are pointless because they usually just change the name of the file. If any one changes it back manually (renaming via a file manager app or say on a computer) the file is no longer hidden.
I tried many free apps that claim to secure your files. But they either don't work, have flaws that a tech savvy person could use to open the files, or are just too complicated to use.
Best Free Recommendation
The absolute best free app I found, use, and recommend is File Locker. It is powerful and so simple to use. You just locate the files, select them, and add a password to lock them. The app encrypts the file and even its file name. So no one can open the file without the password. Any one who is snooping and finds the very long encyrpted names can't do anything to open the files. The best they can do is see that there are some files you locked---but only via a file manager app.
Note: Its so powerful, if you forget your password, you are screwed. But that is a small price to pay for the protection you get.