1.5`payload`

This will probably be the thing you reference most frequently.

payload means:

the current body/data being processed.

For an incoming request:

POST /customers

{
  "firstName": "John",
  "lastName": "Smith",
  "email": "john@example.com"
}

Initially:

payload =
{
    firstName: "John",
    lastName: "Smith",
    email: "john@example.com"
}

Then you execute Salesforce Query.

Suddenly:

payload =
[
   {
      Id: "003ABC...",
      Email: "john@example.com"
   }
]

Why?

Because many Mule processors replace the payload with their result.

This is extremely important when writing integrations.