I'd been drawing as a hobby for a solid 10 years at least before I finally had the concept of composition explained to me by a friend.

Unlike the spatial reasoning we delve into here, where it's all about understanding the relationships between things in three dimensions, composition is all about understanding what you're drawing as it exists in two dimensions. It's about the silhouettes that are used to represent objects, without concern for what those objects are. It's all just shapes, how those shapes balance against one another, and how their arrangement encourages the viewer's eye to follow a specific path. When it comes to illustration, composition is extremely important, and coming to understand it fundamentally changed how I approached my own work.

Marcos Mateu-Mestre's Framed Ink is among the best books out there on explaining composition, and how to think through the way in which you lay out your work.

Illustration is, at its core, storytelling, and understanding composition will arm you with the tools you'll need to tell stories that occur across a span of time, within the confines of a single frame.