The land described herein shall not be deemed to include any house trailer or mobile home standing on said premises.
The land -> a piece of land described herein -> described in this document shall not be deemed to include -> shall not be seen as land + something else"
That "something else" is "any house, trailer or mobile home"
standing on said premises. -> that is standing on that piece of land.
In a simple way:
When we talk about "the land" in this document, we mean just the land, not any house, trailer or mobile home that is on that land.
or very short:
We are talking about the land without any of the mentioned things on it.
It means the land being described shouldn't be considered or assumed to contain a house trailer or a mobile home within its boundaries.