This is a water based coating that protects printed pieces and provides a surface that deters dirt and fingerprints.
Unlike varnish that requires an entirely different process after print, a water based coating is applied, like ink, while printing. Aqueous coating also dries faster than varnish, allowing your print shop to turn around jobs faster. The ease of using water based coatings vs. varnishing reduces costs and speeds up production. Additionally, aqueous coating is odorless and can be better for eventual recycling.
Coating comes in four main types: sealer, gloss, matt, and silk. Rule of thumb is to use matt coating on text heavy products to reduce glare for enhanced legibility, while gloss makes graphics shine. Matt or silk coated paper highly benefit from this protective layer as they smudge more than gloss coated paper. To highlight specific areas of your project, i.e. spot varnish, oil based varnish give superior results.