A 'subdivision' is a tract of land that is subdivided into smaller lots to be sold, typically for houses to be built on them.
For example. I buy a 4 acre lot of land, and then decide to subdivide it. I turn the 4 acre lot into 16 quarter-acre lots for sale.
A subdivision is also a term we use to describe a tract of land that has already been built on and subdivided.
In the United States, subdividing lots would be subject to local zoning laws.