What is meant by taking dual of a boolean expression?
The dual of a boolean expression is an expression that is obtained following these steps:


replace any logical sum (OR) with a logical product (AND)
replace any logical product (AND) with a logical sum (OR)
replace any 0 constant with a 1 constant
replace any 1 constant with a 0 constant


The two basic logical operations AND and OR are called dual of each other, i.e. AND is the dual operation of OR and viceversa. Hence the dual transformation can be explained as:

Replace any operation with its dual and any constant with its complement (negation).

Note that negations must not be changed.