tx is the human-readable interpreted unsigned transaction. Its purpose is to make it easier for the implementer to understand what the content of the transaction is. It does not contain the structure that can be serialized directly (e.g., [addresses] may be represented as Bech32-encoded strings while in the encoded transaction, these would be binary blobs).
signed_tx is the human-readable signed transaction to make it easier for the implementer to understand how the signature envelope looks like.
encoded_tx is the CBOR-encoded (since test vectors are in JSON and CBOR encoding is a binary encoding it also needs to be Base64-encoded) unsigned transaction.
encoded_signed_tx is the CBOR-encoded (since test vectors are in JSON and CBOR encoding is a binary encoding it also needs to be Base64-encoded) signed transaction. This is what is actually broadcast to the network.
valid is a boolean flag indicating whether the given test vector represents a valid transaction, including:
transaction having a valid signature,
transaction being correctly serialized,
transaction passing basic static validation.
NOTE: Even if a transaction passes basic static validation, it may still not be a valid transaction on the given network due to invalid nonce, or due to some specific parameters set on the network.
signer_private_key is the Ed25519 private key that was used to sign the transaction in the test vector.
signer_public_key is the Ed25519 public key corresponding to signer_private_key.