LR thinks that two things have happened: You've changed the metadata for the photo within LR (caption, keywords, develop settings, etc.), and another application has changed some metadata settings in the photo's file. You can either keep LR's metadata changes or the application's metadata changes but not both. If you choose "overwrite", you're keeping the changes made in LR; if you choose "import", you're keeping the changes made by the application.
Sometimes LR gets confused and reports these metadata-changed messages even when there's no external application involved; in these cases, it's always save to choose "overwrite".
Thanks. The files I have been getting this on have not been used for a while and haven't been used with any other programs, so it wasn't making any sense to me at all. Overwrite it is, then. Thanks, again.