Specifications tell you what a tennis racquet or string can actually do. Compare weight, balance point, stiffness ratings, and materials with tested breakdowns.
Here is a claim you have probably heard, maybe from a hitting partner who buys whatever can is cheapest: a tennis ball is a tennis ball. They all bounce. They all wear out.
Open a fresh can and you hear the same hiss every time. That sound is the most quoted fact in the sport: tennis balls are pressurized, and the pressure is what makes them bounce.