Here's what happened:
* Bellatrix, a Death Eater and Draco's aunt, was furious with him for failing to kill Dumbledore. She demanded to know why he didn't follow through with the mission and what he was hiding.
* Draco, fearing for his life, lied and said he was going to lure Harry Potter to the Room of Requirement, where he would kill him. He knew this was impossible, as he didn't have the strength or courage to kill Harry.
* He hoped this lie would satisfy Bellatrix and buy him some time. He was desperate to avoid her wrath and her suspicion that he might be a traitor.
This lie eventually led to a confrontation between Harry and Draco in the Room of Requirement, further complicating the events of the story.