Now the APPEND function, which adds strings together, and the CONCAT function, which adds arrays together, can take more than two arguments.

This means that instead of writing:

APPEND("a", APPEND("b", "c"))
CONCAT([1, 2], CONCAT([3, 4], [5, 6])

You can now write:

APPEND("a", "b", "c")
CONCAT([1, 2], [3, 4], [5, 6])

Read the docs for multiple examples of applying these functions: APPEND and CONCAT