Drag a selection box over a live preview of your PDF's pages to trim margins or unwanted edges. The box you draw is the part that's kept — everything outside it is cut away, the same as cropping a photo.
For keeping — the box you draw is what stays; everything outside it is cropped away. That's how cropping works everywhere (photo editors, Word, phone cameras), not specific to this tool.
Yes — choose "Only page N" above the preview before drawing your selection. It always targets whichever page is currently shown, so if you navigate to a different page while that option is selected, it retargets there too. Choose "All pages" to apply the same crop everywhere, which works well for documents with a consistent layout.
Yes — just drag a new selection on the preview; it replaces the previous one before you click crop.