It's complex, I'm afraid.
It usually applies when your employer is entitled within the employment contract to lay you off or put you on short time working. If he does this to you for a long enough period, you can claim that he is making you redundant. You would put a claim in for redundancy. If the employer does nothing, you can assume he accepts your claim. He may write to you with a 'counter-claim', saying that work will be available again shortly and he is denying your redundancy claim.
Read about it here and post again if you still don't understand, or your situation is different. If you are in a Union, use it.
http://www.direct.gov.uk/en/Employment/Redunda ncyAndLeavingYourJob/DG_10037790