Question on a will

I have a question about a section of a will it reads as follows: Wherever
the words \"my Executor\" or \"my Executors\" or words of reference to them
(such as \"his\" or \"their\") are used in this, my will, such words shall
be deemed to mean, refer to, and include my Executor, and my substitute
Executors hereinabove named, the survivor or survivors of them, any
administrators with the will annexed, and each of their successors or
successor.

[Postman]

Where's the ?

What does this section mean? Who are the successors??