The Keystore is an encrypted version of your private key in JSON format. It is a fancy version of your private key that is protected by a password of your choosing. When this Keystore is used in conjunction with a password, it resembles the private key. The keystore is safer than the private key because you need a password to access it. When you are creating your password make sure it is strong and not the same as your email, or other social media account passwords. It should instead be a new password that has a mixture of symbols, letters and numbers.
The Keystore on imToken is a string and not a JSON text file. If you want to import another wallet using a Keyfile, you need to open and view it in a text editor. Then, copy and paste that to imToken under 'Keystore'. Do not alter any of the information in the text file.
Here is an example of what a Keystore looks like:
Remember: Make sure you backup your Keystore regularly! Only if you remember your password, you can re-import your Keystore again. imToken cannot reset your password or retrieve your keystore. The keystore is only on your phone, you must keep it in a safe place outside of your phone. Make sure it is a place that only you can access.