So before I start if you don't have es file explorer, install it from google play store.
To encrypt your file(video, mp3, txt, avi, mp4 anything at all), follow this simple steps.
Step1 Open es file explorer ==> then locate the file you want to encrypt
Step2 mark the file(hold on the file with your finger until the mark sign pop's up) ===>then click more==> click encrypt
Step3 Set the password you want for the file
Step4 then click okay,
Step5 input your password anytime you want to use/view your encrypted file.thats all