None of this is needed to send a request. It is here for when a term in a response is unfamiliar, or when a decision needs context the reference cannot give.
If you are here to integrate, start at Quickstart.
How card payments work
The parties in a card payment, and where the gateway sits among them.
Card networks and interchange
Where the fee goes, and the request fields that move it.
Tokenization
Why a payment token is worth using.
AVS and CVV
Two fraud signals, their result codes, and what to do with them.
PCI DSS and your scope
How much compliance obligation you carry, and how to carry less.
ACH and NACHA basics
How bank transfers differ from cards, and why returns arrive late.