C-band is all frequencies between 4 and 8GHz. This frequency had been used for satellite TV since the 1970s, but as C-band satellite reception requires “big, ugly dishes” up to 10 feet in diameter, it has been largely replaced by more flexible systems with smaller dishes. (think Dish and DirecTV). Since we have more advanced methods of digital encoding than were available in the 70’s, the satellite companies can now “repack” their broadcasts into the upper portion of the C-band, leaving the lower portion available for cellular companies to use.
Some carriers are delivering 5G that’s either no faster than LTE would be on the same frequencies or has poor range. C-Band can fix this, offering plenty of bandwidth for several different wireless carriers to have solid 5G using mostly existing cell sites. The availability of this newly licensed spectrum won’t be immediate but will happen in waves over the next few years. Once the first block of C-Band frequencies come online, you can expect to see dramatically improved speeds for 5G over much wider coverage areas.