Black-Scholes assumes one volatility for everything. Real markets quote a different implied-volatility for every strike and every expiry, and plotting them produces a surface with two visible features: volatility-skew across strikes and volatility-term-structure across time.
The surface is the working object for anyone trading volatility rather than direction. Positions are described by where they sit on it, risk is measured by how it can deform, and most professional edge comes from spotting a point that is cheap or rich relative to its neighbours.
Example: XYZ at $50. The 30-day $45 put trades at 31% implied volatility, the 30-day $50 at 25%, the 30-day $55 at 23%, and the one-year $50 at 27%. Those four numbers are four points on the surface, and together they already tell you the skew is steep and the term structure upward sloping.
Related: volatility-skew, volatility-term-structure, volatility-smile, implied-volatility