Canny Edge detection makes use of Hysteresis, a technique which uses a low-threshold and high-threshold to make decisions regarding which edge pixels to include within the final edge image.
Hysteresis
Values greater than our high-threshold are automatically included within our final edge image. Values lesser than the lower threshold are automatically excluded from our final edge image. Values that are greater than our lower threshold and lesser than our high threshold (aka between the thresholds) are only included if it is connected to an edge above the high threshold. The idea is to continue our edge with this technique