What exactly is the payload?

Question

I am trying to understand what is in a transaction specifically the payload. While the documentation references the term payload, it is not explicit/clear about what exactly it is. It seems to include the block number and block hash for determining mortality but what else does it include? I need an "explain payload to me like I was 5", I can sort out the details but I need a clear, conceptual model of what is in the extrinsic/transaction. Furthermore, the payload is then signed and then serialized. Was it not serialized before being signed?

This is the documentation I have looked at: https://wiki.polkadot.network/docs/build-transaction-construction

Is the payload, this information? If so, the metadata is already encoded, so does it get encoded again?

Address: The SS58-encoded address of the sending account.
Block Hash: The hash of the checkpoint block.
Block Number: The number of the checkpoint block.
Genesis Hash: The genesis hash of the chain.
Metadata: The SCALE-encoded metadata for the runtime when submitted.
Nonce: The nonce for this transaction.*
Spec Version: The current spec version for the runtime.
Transaction Version: The current version for transaction format.
Tip: Optional, the tip to increase transaction priority.
Era Period: Optional, the number of blocks after the checkpoint for which a transaction is valid. If zero, the transaction is immortal 

Answer 1

In the page https://wiki.polkadot.network/docs/build-transaction-construction

payload refers to the entire object which must be signed and submitted with the signature to the node.

The payload may have different things in it depending on the configuration of the Substrate chain, which makes it very flexible for developers, but of course hard to explain here.

If that is the case, you can look at the answer here: What are the Steps for Encoding an Extrinsic in Substrate?

Answer 2

In simple terms, payload is just a chunk of data carried with transmission unit. You can go through this link which has awesome description of payload concept.