All Stellar accounts must maintain a minimum balance of lumens. The minimum balance is calculated using the base reserve, which is currently 0.5 XLM:
Minimum Balance = (2 + # of entries) * base reserve
The absolute minimum balance for an account is 1 XLM (which can not be moved out), which is equal to (2 + 0 entries) * 0.5 base reserve.
Memo contains optional extra information for transaction. It is usually REQUIRED when sending funds to crypto exchanges or anyone else that explicitly asks you to use it (exchange will provide you with information) to identify the receiver in a Stellar transaction. It is not required when you send XLM funds to your BC Vault XLM wallet.