A drone is a note or chord held throughout the duration of a piece or group of other notes. These drones prove useful for practicing intonation. It teaches your ear to hear not only the note the drone is sounding, but also to recognize the intervals in the key or scale.
However, like any other practice, this requires slow and focused listening for slightest deviations in intonation.
For this exercise, you can use the Fiddlerman Drone Tool.
You can practice scales, études, and repertoire with drones. Select the key of the drone, and begin a slow and methodical work session.
It's best to practice without vibrato and to repeat the intervals for memorization.
Watch Fiddlerman demonstrate below.