Croquis' are fun little pattern ideas that textile designers generate before turning them (well, the good ones anyway) into full pattern designs.
Textile inspiration can start with anything really. But normally, for me, it begins with either an illustration, croquis (which is a small test pattern) or photo. Illustrations, especially fashion ones give a lot more information than the other options. This can be really helpful in deciding things like fibre, density of the fabric, and even how much detail may or may not be added. Like any form of design outcome (and probably like most things in life?) the more you know, the better off you are.