Shirring, ruching and gathering are things done to fabric to add texture, fullness, shape and design elements.
Ruche means a pleated, fluted, or gathered strip of fabric used for trimming.
Shirring is formed by multiple rows of gathers and is a way to create controlled
fullness, like at a waistline, cuffs or a bodice yoke. It’s often accented with decorative stitching or smocking and stabilized so the multiple thread rows aren't stressed.
Gathering is drawing up fullness into a predetermined size smaller area.
Angela Wolf shows how to ruche a shirt here.
Patternmaking books are filled with illustrations and information to teach you how to add these types of design elements to your own or existing patterns.