Retouch varnish is a temporary varnish you can apply to an oil painting when it is touch dry rather than fully dry. Unlike 'normal' or 'final' varnish, retouch varnish allows an oil painting to continue to dry. When the painting is eventually dry, you can apply a final varnish over the retouch varnish (be sure to brush any dust off first!).
Retouch varnish evens out the gloss of a painting, and protects the painting if you want to exhibit or ship it.