What are private key, keystore file and mnemonic phrase?

If you can't see tokens that were sent to your wallet's address then most probably you need to add it to your wallet.

Private key

This is the unencrypted text version of your private key. It doesn’t require a password to access your wallet.
Note:
    - Anyone can use it to access your wallet, if they were to find your unencrypted private key. For this reason, encrypted versions are recommended.

Keystore file

A keystore file is an encrypted text version of your private key. It's generated by using your private key + a password of your choosing. To access a wallet address with a keystore file you need to provide a valid password.
Note:
    - You cannot decrypt a keystore file without the password, and there is no way to recover or reset that password.

Mnemonic phrase

A mnemonic phrase is actually a whole bunch of private keys, represented by a 12- or 24-word phrase. To access your wallet you need to type all of the words.
Note:
    - Mintme.com desktop’s wallet uses 12-word phrases