C-band is a broad name used for part of the microwave radio spectrum, with definitions that vary by industry and application. In US mobile-network discussions it commonly refers to the lower 3.7 GHz band used for 5G, while satellite services have long operated in adjacent frequencies.
US regulators made the 3.7–3.98 GHz range available for licensed terrestrial wireless use after arranging to move affected satellite operations. The transition illustrates why spectrum repurposing involves more than a new radio standard: incumbents, interference protection, auction rules and installation work all matter.
Mid-band frequencies are attractive because they can balance usable coverage with more capacity than many lower-frequency bands. The result still depends on cell density, antenna design, spectrum holdings, devices, network load and local building conditions.
When you see a proposal to reuse C-band for 6G, check which country's definition and frequency range it uses. The fact that 5G operates there today does not prove that 6G equipment can be enabled through a software update or that regulators have authorized every proposed use.