Metadata is information that is included in the file that provides information about the file. Simple examples include things like the date and time the image was created, the camera or software used to create the image, the orientation of the image, or copyright information about the image. More complex metadata types include things like ICC color profiles or processing information for image editors.
The WebP format does include support for metadata such as EXIF, XMP, and ICC. It can support IPTC, although that's not common. In general, WebP is designed to be lean, and extra metadata information embedded within the file works against that (which is why many image optimization methods strip out image metadata).
You can find much more technical information about how metadata is stored and structured in WebP here.