With test screenings, an early version of the film is shown to a small group, and then opinions are gathered from that group. The studio sees what they liked, what they didn’t, and might invest in reshoots or another round of editing to fix some of the problems that have been perceived.
Article source: https://www.cinemablend.com/news/2485858/surprise-star-wars-the-rise-of-skywalker-didnt-have-any-test-screenings