Application Password
To this day, there is no way to password-protect apps on iOS without jailbreaking. If you are familiar to rooting an Android device, it is somewhat similar to jailbreaking. If you already have jailbroken or want to jailbreak your Apple device, check out this link. If you decide to jailbreak, you can get password-protecting features through tweaks such as bioprotect or appscan if you have touchID. If not, there are similar tweaks that offer the same password-protecting features. If you aren't wanting to jailbreak your device, there is no other way. Hope this helps!