When you click on the "Send to Atomic" button in Atomic Wallet funds from your address is being transferred to Atomic Wallet address.
For example, if you want to import your Ethereum private key - all your funds from your Ethereum address are being transferred to Atomic Wallet ETH address. (except network fees). You will have one Ethereum address instead of two.
In the future we will probably add changes to the function.
Atomic Wallet do not support mnemonic seed import. For now our product supports only asset's private key import.