FT8 is a weak-signal mode for making quick contacts, even when signals are barely audible or the antenna is modest. Instead of free conversation, you exchange a short, fixed series of messages in timed slots, and the computer decodes them from below the noise. FT4 is its faster sibling, aimed at contests. Both come with the WSJT-X software.
How it squeezes data out of noise
FSK: the bit picks the frequency. PSK: the bits pick the phase. QPSK packs two bits per phase.
Each message is tiny, so it can be packed with heavy forward error correction and sent slowly relative to its length. FT8 sends one of eight tones at a time, the tone chosen by the data. A transmission occupies about 13 seconds of the 15-second slot. The receiver uses the error-correcting code to find the message that best fits what it heard.
The signal report in an FT8 exchange is the decoder's estimate of the signal-to-noise ratio measured in a 2.5 kHz bandwidth, not an S-meter reading. Negative numbers are normal; a report of +3 means 3 dB above the noise in that reference bandwidth.
A contact, slot by slot
One message per slot, alternating. Example call signs and reports are illustrative.
Each station transmits in one slot and listens in the next. Typically you see the call, then grid squares, then reports, then a closing 73. The fixed format is what lets the decoder be so sensitive, and also why you cannot chat freely (see JS8Call for that).
Timing and where to answer
Time slot = who talks now. Waterfall frequency = where. Answer in the other slot, on a clear spot.
Slots begin at fixed times, so your computer clock must be accurate to about a second. FT8 does not synchronise to the other station: everyone's clock is the reference. Software decodes every signal in the passband at once, so the waterfall shows many stations at different audio frequencies.
To answer a CQ, pick an empty spot on the waterfall and transmit in the alternate slot from the station you are answering.
In practice
Use USB, and turn the audio level down until the radio's ALC barely moves; an overdriven signal spreads across the waterfall and annoys neighbours.
FT8 keys the transmitter for most of each slot, so run well below the radio's rated power.
FT4 trades some sensitivity for a quicker exchange, which suits contests. In VHF contests the exchange uses the grid square instead of a signal report.