Vishistam means together, Advaitam means one.
The essence of Vishistadwaitam is that Paramatma Narayanan alone is together with the universe and all Jeevatmas.
It does not mean that Perumal and the universe are separate.
Perumal is omnipresent in all the souls and materials; there are no materials that are separate from Him. He is the supreme soul in all.
We shall take the tree as an example. There are leaves, fruits, flowers in a tree; besides these, there are other qualities of a tree such as its scent, taste, and other matters that make up a tree. We call this whole entity as a tree. Similarly, Paramatma Shriman Narayanan is together with all sentient beings and insentient universes; this is Vishista Advaidtam.
This can be learned extensively through kalkshepam-s and by reading in books.