Patterning is the use of multiple masks to get fine features smaller than the wavelength of light you're using where in previous larger feature nodes one mask sufficed: https://en.wikipedia.org/wiki/Multiple_patterning

It's very expensive in terms of tooling, since you need a lot more more masks, and production time spend in lithography steps. See this comment in this discussion https://news.ycombinator.com/item?id=19570724 for a bit more.