The Ghostlight is a haunted legend, described to be a blue light that haunts around Ornament Valley.
In Cars, after Mater and Lightning McQueen have spent McQueen's third night in Radiator Springs going tractor tipping, Mater considers searching for the Ghostlight the following night. When the press arrive in Radiator Springs after learning about McQueen's presence, Mater believes a spotlight shining on McQueen is the Ghostlight, but it turns out to be Kathy Copter's searchlight.
Mater and the Ghostlight
In Mater and the Ghostlight, Mater originally jokes about the Ghostlight, though Sheriff warns him of this, telling the Ghoslight's story. After the story, everybody goes to sleep, leaving Mater alone. He then travels and notices a blue light in his wing mirror, believing it to be the Ghostlight before driving off scared and travelling through the entire town until he meets with the gang again, and they reveal that the "Ghostlight" was a blue lamp hooked onto Mater.
Cars: Mater-National Championship
In Cars: Mater-National Championship, when McQueen arrives in Ornament Valley, Mater comes to alert him about some lights shining around the valley as well as a strange sound. He fearfully assumes that it is the Ghostlight, and heads off when he, McQueen and Sarge hear a peculiar sound. Later, McQueen tries introducing Gudmund to Mater, but Gudmund's big number of headlights makes Mater think he is the Ghostlight, while Gudmund also believes Mater is the "Haunted Hook", causing both of them to drive away in fright.
In the cutscene for Ghosting Mater, after Sheriff finds out that the tractors have wandered off into Radiator Springs again, as a result of Mater and McQueen going tractor tipping, he plays a trick on Mater by reminding him of the Ghostlight and placing a lamp on his hook when he is not looking. On the way back to his home, Mater, believing the lamp is the Ghostlight, gets frightened throughout his journey.
In the cutscene for Smash Up 2 in Cars: Race-O-Rama, when bluffing that he had not been scared by Bubba, Mater brings up the mention of the Ghostlight, describing it to "shake the rust off your bumper, the dirt from your windshield, the holes from your carburetor!"