Because the industry has largely moved to digital processes, the job of stripping, or planning has become rare or even obsolete.
A screened negative has varying sizes of dots arranged in a particular pattern to represent greater or lesser density of image.
"Cleaning" a negative of unwanted artifacts (noise) from the line camera process, (lines, etc., from the edges of traditional hot-wax analog "font" paste-up, and dirt), would be done by scraping the exposed emulsion from the negative with an X-Acto Knife (for areas intended to be exposed on the printing plate).
The painstaking alignment of the negatives requires a stripper to use high-magnification eye piece while viewing the work on top of a light table to achieve exact positioning.
The plate is then developed and the unexposed areas are washed away, leaving behind cured emulsion that will hold the ink on press.