atomic locking is a technique to ensure the cross-shard transaction are consistent between shards. It’s mainly used for the case where you need to send token from 2 shards at the same time. It works as first the tokens from the two shards get locked, and then those token will be spent together on the destination shard, and finally the token from the two originating shards gets unlocked and deducted.
A keystore file (sometimes called a UTC file) is an encrypted version of your private key. They are generated using your private key and a password that you use to encrypt it. If you open up your keystore file in a text editor it contains data pertaining to the encryption of the private key. more information are available here : https://medium.com/@julien.maffre/what-is-an-ethereum-keystore-file-86c8c5917b97