Parts generates a Parts List by summarizing all visible components in the active scene.
This page includes the filename, the scene name and the units used in the model. A summary section lists all parts, grouped by material and thickness with quantities and some rough dimensions depending on the type of material. From the summary you scroll down to the desired group.
To shorten a long Parts List, you can hide the summary and all groups, up to the title bar.
Generate will recompute the Parts List after a change in the model, after the selection of active parts or the current scene has changed. If you get annoyed by OpenCutList always asking to regenerate the Parts List, you can click on Ignore or simply minimize the window. Because of the dynamic nature of the list, it is important for you to know if the list is in sync with the model and your selection.