Vertex
A single point in 3D space, the smallest building block of a mesh.
A vertex (plural vertices) is one point in 3D space. On its own it renders as nothing; its job is to anchor edges and faces. Moving vertices is the finest-grained way to shape a model. Blender shows vertex counts in the statistics overlay, and low-poly game assets are often described by their vertex or triangle count.
These fundamentals are covered hands-on in Blender Geometry Essentials 01, free with a free account.