Lightroom by default saves the work you do in Lightroom to the LR Catalog file. If you have the option in your preferences selected to save metadata to .xmp Lightroom will also write data to an .xmp file.
If you use an external editor like ACR in Photoshop CS or Elements to make changes to the raw file then there will be a mismatch between the info in the LR Catalog and the .xmp file. (hint. ACR does not read or write info to the LR Catalog file)
Lightroom is therefore prompting you to decide if you require the info in the Catalog file to be updated. (hint. changes made in ACR or other applications will not automatically update the info in the LR Catalog.)
Lightroom procedure is to read and write to the Catalog, reading and writing to the .xmp file is a user option.