Loop Cut
A new edge loop inserted across a ring of quads.
A loop cut inserts a connected line of edges across a quad ring, adding resolution exactly where you need it. In Blender it lives on Ctrl+R. Loop cuts are how you tighten corners for subdivision and add detail without disturbing surrounding geometry.
Related: Edge Flow · Quad · Subdivision Surface
These fundamentals are covered hands-on in Blender Geometry Essentials 01, free with a free account.