In the context of Rosette, a threshold determines a boundary. When a threshold is defined, only results with scores above the threshold are returned. Determining the correct threshold value allows you to see all the positive results of interest, without extraneous negative results. Increasing the threshold removes false positives, but if the threshold is set too high, true positives may be removed as well.
While Rosette may have a default value defined for a threshold, the optimal value will depend on your input text, data, and the problem you are solving.