Edit Mode
The Blender mode for editing a mesh's vertices, edges, and faces.
Edit Mode is where actual modeling happens: you select and manipulate the components of a mesh (vertices, edges, and faces) rather than the object as a whole. The shortcut to switch between Object Mode and Edit Mode is Tab. The process of manipulating components in Edit Mode is called modeling.
Related: Object Mode · Mesh · Modeling
These fundamentals are covered hands-on in Blender Geometry Essentials 01, free with a free account.