This analysis takes a large table, and creates as output a smaller table, which can be easier to work with. But note that your original data are still part of the file, which means the project file gets larger not smaller. But it can be useful to have a smaller data set for quicker analyses and graphing.
To prune, click Analyze and choose Built-in analyses. Then choose Prune from the list of data manipulations to bring up this dialog.
Note that Prism can also decimate while importing data – import a row, skip K rows (you enter K), then import another row.