GVFS provides a layer just below the user applications you use like firefox. This layer is called a virtual filesystem and basically presents to firefox, thunderbird and pidgin a common layer that allows them to see local file resource and remote file resource as a single set of resources. Meaning your access to the resource whether on your local machine or the remote machine would be transparent to the user.
Although this layer is mostly there to make it easier for application developers to code to a single set of interfaces and not have to distinguish between local and remote file system and their low-level code.
For the user this could mean that the same file manager you use to browse your local files, could also be used to browse files on a remote server. As a simplified contrast, on Windows I can browse my local files with Explorer, but to browse files on an NFS or SFTP server I would need a separate application.