Why FT8 is one of the WSJT-X modes; the "8" in the name refers to 8-tone frequency shift keying (8-FSK). The transmitter steps among 8 tones spaced 6.25 Hz apart, giving a signal only about 50 Hz wide, and each 15-second transmission carries a short structured message. Because it is constant-envelope FSK, it can be sent through a transmitter run at full output without distortion concerns.
Watch out Spread spectrum is tempting because it also mentions 8, but FT8 is extremely narrowband, the opposite of spreading energy over a wide bandwidth; vestigial sideband is a legacy analog TV video technique.
The 8 in FT8 = 8 tones. 8-FSK, 8 tones, 6.25 Hz apart, 15-second slots.