A visual analogue scale is a system to rate a person's level of a particular symptom along a line drawn between two fixed points.
The person, for example, is told that the one end of the line is 'no pain at all' and at the other end is 'unbearable pain' and then they indicate on the line where they think there pain fits.
Validation of Digital Visual Analog Scale Pain Scoring With a Traditional Paper-based Visual Analog Scale in Adults. Delgado DA et al. J Am Acad Orthop Surg Glob Res Rev. 2018 Mar; 2(3): e088.