Fidelius Charm / How Potters died

'You can imagine how the idea would torment him.'
'Because Black turned out to be in league with You-know-Who?'
'Worse even than that, m'dear... Not many people are aware that the Potters knew You-Know-Who was after them. Dumbledore, who was of course working tirelessly against You-Know-Who, had a number of useful spies. One of them tipped him off, and he alerted James and Lily at once. He advised them to go into hiding. Well, of course, You-know-Who wasn't an easy person to hide from. Dumbledore told them that their best chance was the Fidelius Charm.'
'An immensely complex spell involving the magical concealment of a secret inside a single, living soul. The information is hidden inside the chosen person, or Secret Keeper, and is henceforth impossible to find - unless, of course, the Secret Keeper chooses to divulge it. As long as the Secret Keeper refused to speak, You-Know-Who could search the village where Lily and James were staying for years and never find the, not even if he had this nose pressed against their sitting-room window!'
'So Black was the Potters' Secret Keeper?
'Naturally,'

Pg 216 - THE MARAUDER'S MAP