The concept
Composition is the arrangement of elements in your artwork. Basically, where you put the objects in your scene and why. The purpose of composition is to shape how the spectator sees your work, or in other words, what you want to draw the viewer's attention to first, what is most important.
In practice
While in 2D art it is one of the essential subjects of study, in 3D it often seems neglected especially at the beginner level.
Since we create scenes in 3D, it may feel like composition is an overwhelming and almost impossible task as the scene can be viewed at almost any angle. Still, one of the first things to keep in mind when getting a concept of a scene together, whether basing it on an existing concept art or creating your own, are camera angles. It is always good to establish your main shot(s), which will help determine what is in your foreground and what you want to focus on (certain props that you know will be in the background won't need detailed 4k textures, for example).
Composition principles are universal, so do not shy away from resources which treat composition in traditional art or concept are, and not 3D art specifically. For the purposes of learning, you can think of your camera shot as a set 2D art work.
Before getting overwhelmed with all the formal rules of composition, ask yourself first what is the most important element(s) of your scene and what you can do to make it stand out from the rest. And remember, you do not need to apply of the rules at once, pick those that you want to explore or that actually make scene for your scene.
Blender Guru does a quick overview of basic composition principles. Concept Art Empire offers some good tips for 2D art, which will give you an idea of the formal principals. Understanding Shapes goes into a more details demonstration of how shapes work in landscape art (if you are working on a outdoors vista type scene, you may find this useful).
As Art Station Learning is free for the rest of 2021, you can also take a look at Stephane Wootha Richard's Composition in Painting is an excellent course, it goes over some principles but also offer a slightly different approach of what is most important. Composition and the Language of Shapes offers a solid breakdown of classic principles of composition.
Learning Resources
What is composition? - McKella Sawyer (Concept Art Empire)
Understanding Composition - Blender Guru
Understanding Shapes - moderndayjames
Composition in Painting - Stephane Wootha Richard (Art Station Learning)
Composition and the Language of Shapes - Jason Scheier (Art Station Learning)