Content is your notes. In obsidian this is represented by markdown files in a flat filesystem. This format is already cross platform and doesn’t need to be exported.
Metadata is extracted information from your notes that makes processing the data more efficient. Tags, links, timestamp, keywords, titles, filenames, etc are metadata, stored in the metadata database. When you search for something in obsidian, or view the graph, or list files in a tag etc obsidian only opens the metadata database to process the request. It only opens the file for read/write.