The Resolve Missing Files dialog shows when you have attempted to load a project file and some of the files were missing.

For each missing file, you will be prompted to Resolve.
If you choose the Resolve option then you will be prompted to browse to and choose a new file for that clip. Note that the new file must be of the same type as the original missing file.

Once you have resolved missing files, click Finish to continue loading the project. Files will be removed from the project if not resolved.