Marrying isn't just a fancy term, it's actually an important step in the production process, it's one of the final stages in creating a great whisky.
Once whiskies from different casks* have all reached maturity, they are combined together in oak marrying tuns*, where they are blended and mellow together.
While this is an 18 year old single malt, it's actually a blend of Glenfiddich whiskies that are at least 18 years old. To obtain the very best product, older whiskies may be added, or whiskies that have been stored in different types of casks, e.g. new oak, charred oak, wine casks &c.
*Cask, tun, barrel, hogshead, pipe, butt, firkin etc are all simply sizes of old-fashioned barrels, made from wooden staves and held together with iron bands, as you'd expect. :)
Thanks @Hayley Morris for sharing the insights.