"Metadata Status" refers - as dj_paige has said - to writing the metadata to file (either true sidecar file or into the header of JPGs, TIFFs, PSDs and DNGs - but not into the image pixels!)
This "writing metadata to file" is a separate and additional saving of the metadata that are already saved in the Lr catalog. Lr saves everything you do in Lr instantly and automatically in its database (= catalog). Since it is not saved in the image pixels - as you probably know - it is called metadata.
In addition to the saving in the catalog there is the option of "writing metadata to file". But this option is not necessary for the function of Lr. It is a feature that enables users to see the Lr edits in Adobe Bridge.
The warning you are referring to means that the data in the catalog and in the file are not identical. Since Lr always uses the catalog (unless you do a "read metadata from file") your photos and the edits are safe and sound.
However, if you want to also save the metadata to file, go to the Catalog panel (just above the Folders panel) and select <All Photographs>. In the Grid View, press Ctrl / Cmd + A (letter A on your keyboard) to select all of your photos. Then press Ctrl / Cmd + S to save the metadata for all selected photos to file. **) This will make the metadata in the catalog and in the file identical - for the moment. As soon as you do further editing in Lr, the metadata in catalog and file will again be different (for the edited photos). Naturally you can repeat the Cmd / Ctrl + S process for the edited photos.
I have this in my workflow: As soon as I have edited a bunch of photos, or at the end of the day, I go to the Grid View, select all newly edited photos and do Ctrl + S.
There is also a feature in Catalog Settings / Metadata tab to <Automatically write changes to XMP>. But I would recommend not to check this box because (a) it causes a constant reloading of TIFFs and PSDs in the Develop Module which is not harmful but annoying; and (b) there is an old bug that causes to send the "metadata warning" message when this box is checked.
**) Depending on the number of photos in your catalog this can take quite a while.