Knitting increase

I am new to knitting. What is Rib 1, *inc in next st, rib 1 (2,1) * dept from *

Welcome to KH and to knitting!
Rib means to work the stitch as either a knit or a purl, depending on which stitch occurs in the previous rows. If it’s a V, knit it; if it’s a bump, purl it.
Increase in the next stitch could be a knit front and back (kfb). There are other increases in a stitch but that one works easily and well.

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Rib 1 (2,1) probably refers to different sizes, S, M, and L. Rib the number of sts for your size.