Before re-saving a picture the tool inspects its JPEG, PNG or WebP container and lists the kinds of metadata it finds: EXIF, XMP, IPTC, an ICC colour profile, and text chunks or comments. It also says whether a GPS location was present and, where available, the camera make and model and the date taken. A typical phone photo might report EXIF plus a GPS position, a phone model and a timestamp, which together can show where and when the picture was taken and on which device. If nothing is found, the note says so and the file is still re-saved clean.
Metadata also covers serial numbers, lens and exposure settings, embedded thumbnails, the editing software used and author or copyright fields. Re-encoding drops all of these, because the browser creates a new file containing only the pixels. Dropping a cleaned copy back in should report that no metadata was found, which is a quick way to check.
Expect file sizes to change. Re-compressing at quality 92 and losing the embedded thumbnail and colour profile often makes the copy smaller, but a result can also come out larger than the source, particularly with PNG output on a photograph. The note under each file shows the before and after sizes, so you can see which happened and switch the format if you prefer.