Transformational grammar, a theory developed by Noam Chomsky, describes how sentences are related through underlying structures and transformations. It posits that sentences have both a "deep structure" (an underlying meaning and representation) and a "surface structure" (the way it's pronounced). Transformations are rules that convert deep structures into surface structures, explaining how different sentences can express the same underlying meaning. 

Here's a more detailed explanation:

Key Concepts:

• Deep Structure: The abstract, underlying meaning and grammatical structure of a sentence. 
Surface Structure: The way a sentence is actually pronounced or written, reflecting its form. 
Transformations: Rules that modify deep structures to create surface structures, explaining relationships between sentences. 
Generative Grammar: A system that generates all and only the grammatical sentences of a language. 

How it Works:

1. Phrase Structure Rules:
These rules generate the deep structure, building up the sentence from basic units like nouns, verbs, etc. 
2. Transformational Rules:
These rules then modify the deep structure to create the surface structure, for example, by changing active voice to passive voice or adding questions marks. 

Example:

Deep Structure (Both Active and Passive):
Britannica states that both "John read the book" and "The book was read by John" share a common deep structure.
Surface Structure (Different):
"John read the book" (active voice) and "The book was read by John" (passive voice) have different surface structures, but they represent the same underlying meaning. 
Transformation:
The passive voice transformation converts the deep structure to the surface structure of the passive sentence. 

Significance:

Transformational grammar provided a framework for understanding how speakers generate and understand an infinite number of sentences from a finite set of rules. 
It was a major shift in linguistics, moving away from descriptive approaches to focus on the generative aspects of language. 
It influenced other fields like psychology and computer science, contributing to the development of theories about language acquisition and language processing.