The spine is the unsharpened top edge of the blade, opposite the cutting edge. Its thickness is the blade stock measurement quoted on spec sheets, and it is the surface you press on when you want more force behind a cut.
Because it is blunt, the spine is where your thumb goes for a controlled push cut, where a baton strikes on a fixed blade, and where a ferro rod gets scraped if the spine has a sharp enough corner. Many outdoor knives are ground with a square spine specifically so it will throw sparks.
Jimping is cut into the spine to stop a thumb sliding. A swedge or false edge is a bevel ground into the spine near the tip.
Spine thickness, quoted as blade stock, is the honest indicator of what a knife was built for. Around 0.1 inches or less is a slicer. A quarter inch is a chopper that will not slice paper-thin.
Thicker is not better. Thicker means more wedge behind the edge, which is strength you pay for in cutting efficiency on every single cut.