This notification indicates that there is a file whose name contains more than 225 characters. Filenames which contain more than the said amount of characters may cause problems on some operating systems.
Since the client synchronizes data from various devices, it may become an issue if the file is transferred to a device which can’t handle file names longer than that. Therefore filenames that contain more than 225 characters should be avoided.
If this notification appears in the client, you simply have to rename the file and select a new, shorter name.