What Is the Significance of : when and where to put the :

Answer 1

I don't know the real context of your question, but I guess you must read 'where' instead 'when'... ( answer is probably 'after the else keyword' if I believe your question tags )

Answer 2

no, they are talking about ":"
You put the ":" when you are about to enter a sub-block of code (i.e. after an if, elif, else, for, while, def, class statement).

It tells the interpreter to look for a indented block after it.