Glossary/Drawing basics
Layer
A named category used to group objects and control their color, linetype and visibility together.
Every object in a drawing sits on exactly one layer. Layers are the primary way to organize a drawing — turning a layer off hides everything on it, freezing it also excludes it from regeneration and plotting, and locking it prevents edits — without having to select objects individually.
Layer names and conventions are often standardized project-wide (e.g. a piping discipline prefix), which is what makes "only extract/plot what's on these layers" a common batch-processing filter.