What is 'immunity' in US Law?

Question

Just browsing over the news stories, came accross the Heath Ledger/Olsen story, that she won't answer questions unless she has immunity. Completely at a loss as to what it means?!

Answer 1

Immunity means she cannot be prosecuted for what she says.

Answer 2

"Immunity" in this case usually refers to an exchange wherein the person testifying (answering questions) gives their testimony under the condition that their testimony won't be used against them.

This kind of immunity is tied to the Fifth Amendment, which protects citizens from self-incrimination via testimony.  The amendment reads that no person shall be compelled to be a witness against him/herself in any criminal case.

Some people invoke this right by saying that they "plead the fifth."