At the beginning of the Deathly Hallows, during a meeting with the Death Eaters, Snape let Voldemort know about the date planned for Harry's departure from the Dursley's.
“My Lord, the Order of the Phoenix intends to move Harry Potter from his current place of safety on Saturday next, at nightfall.”
The interest around the table sharpened palpably; Some stiffened, others fidgeted, all gazing at Snape and Voldemort.
“Saturday... at nightfall,” repeated Voldemort. His red eyes fastened upon Snape’s black ones with such intensity that some of the watchers looked away, apparently fearful that they themselves would be scorched by the ferocity of the gaze. Snape, however, looked calmly back into Voldemort’s face and, after a moment or two. Voldemort’s lipless mouth curved into something like a smile. “Good. very good. And this information comes—”
“—from the source we discussed,” said Snape.
(Deahtly Hallows, Chapter 1, The Dark Lord Ascending)
He followed in this Dumbledore's order:
“You will have to give Voldemort the correct date of Harry’s departure from his aunt and uncle’s,” said Dumbledore. “Not to do so will raise suspicion, when Voldemort believes you so well informed.
(Deahtly Hallows, Chapter 33, The prince's tale)
But he can certainly not tell Voldemort the truth (Hi, I followed Dumbledore's order, he really wanted to make sure you would trust me - no, definitely no).
And surely Snape cannot pretend to Voldemort he's still in the Order's inside track? Even with a lot of Occlumency, Voldemort could not believe the Order still trusted Snape and invited him to its meetings after he killed Dumbledore and flew with Death Eaters.
So what is it that Snape told Voldemort his source was?
How was it more reliable, how did it let Snape become more important and trustworthy to Voldemort than any other Death Eaters who could spy on the Ministry?
( For instance, Yaxley had heard otherwise spying in the Minitry; the whole thing is too long, but this quote shows how trustworthy the "discussed source" seems to Voldemort:
“My Lord,” Yaxley went on, “Dawlish believes an entire party of Aurors will be used to transfer the boy—”
Voldemort held up a large white hand, and Yaxley subsided at once, watching resentfully as Voldemort turned back to Snape.
(Deahtly Hallows, Chapter 1, The Dark Lord Ascending) )
Some questions are linked to this one, especially How did Snape know about the plans to move Harry? (and its dup) and Why did Snape need to appear well informed?. Mine asks only what Snape told Voldemort in the mentionned discussion: 'I have a good source, it's ***'.