What is the significance of the & and && operators in PL SQL ?

If you mean in SQL*Plus, then & and && are for substitution variables. If you use a single & then the value will be prompted for each time it's encountered. IF you use && then the original substitution value is used for the duration of the script

The & operator means that the PL SQL block requires user input for a variable. The && operator means that the value of this variable should be the same as inputted by the user previously for this same variable.

It's actually part of SQL*Plus - sort of a macro thing. Take a look at this:

SQL*Plus Helps

It allows you to - sort of - define a macro that prompts for values to be searched when you execute a particular statement.