Transfers are movements of value from one address to another. In order to figure out how much available currency is present at an address, you'd tally the transfers associated with that address.
Transfers in UTXO blockchains (Bitcoin & Bitcoin Cash) do not normally have a simple from/to - they come in more complex sets. In this case, we use the to_address or from_address "unknown" to indicate this. For example, if you consider a normal two-in-two-out transaction - there will be 4 transfers, two from normal addresses, to "unknown", and two more transfers from "unknown" to the out addresses.