The lecture begins with an introduction to the concept of ambiguous grammar.

• A grammar is ambiguous if a string can have two or more left derivation trees.
• The video references the previous lecture on derivation trees and the process of deriving strings from a grammar.

The concept of ambiguous grammar is defined and explained in detail.

• Ambiguity in grammar is not about having one left and one right derivation tree but two or more left derivation trees for the same string.
• The lecturer emphasizes that only left derivations are considered when determining ambiguity.

An example grammar is introduced to demonstrate the concept of ambiguity.

• The grammar consists of a non-terminal symbol 'S', terminal symbols 'A', 'B', '+', and '*', with 'S' being the start symbol.
• The production rules include 'S' producing 'S plus S', 'S into S', 'A', and 'B'.

The process of deriving a string from the example grammar is shown to illustrate ambiguity.

• The lecturer demonstrates how to derive the string 'A plus A into B' using the provided grammar and left derivation trees.
• Two separate derivations are shown, each starting with different production rules but resulting in the same string.

The video concludes with a reaffirmation of the grammar's ambiguity.

• Since the string 'A plus A into B' can be derived in two different ways using left derivation trees, the grammar is confirmed to be ambiguous.
• The lecturer recapitulates the example to ensure clarity for the viewers.