A secret keeper is someone who knows the whereabouts of a place, usually a house, which has been put under the Fidelius charm (I think that's the name).
Nobody can see such a place unless they have been shown/taken to the place by the secret keeper, or someone who already knows the place's wherabouts.
Grimwauld Place's secret keeper was Dumbledore, but certain events at the end of HBP meant Death Eaters were able to see it in DH.
Or you can just click the link above for a more considered, and factually correct explanation.!