Gear inches equal chainring divided by cog, times the wheel diameter in inches. The diameter comes from the tire circumference divided by pi. A 700c x 25 is treated as 2,105 mm around, which is about 26.4 inches across. A 50/11 on that wheel is near 120 gear inches. A 34/28 is near 32.
Development is the same idea in meters: how far the bike rolls in one crank revolution. Two gears within 3 percent of each other are marked as close, which is the overlap you feel on a compact double.
One gear inch number is free. $5 unlocks the whole cassette, including the close pairs.
Gear is a rollout chart from the wheel and the teeth you enter. Tire pressure, wind, and a worn cassette change the real speed. It is not a fit session and it does not size a frame.