All Stellar addresses are required to keep a minimum balance of XLM at all times. The current minimum is 1 XLM, which is set by the Stellar network.
The minimum balance requirement is in place in order to prevent people from overwhelming the network and to aid in prioritization. The first use of a new Stellar address must be to receive at least 1 XLM.
This will activate the address and allow it to be used in further transactions on the network. You will never be able to send your entire balance from your Stellar address. That 1 XLM minimum will always have to be kept as a balance.