Flagging posts are situated at intervals along the track and are usually in visual contact with each other. At each post is a flag marshal who has a selection of flags that may be shown to riders to communicate important (usually safety orientated) information. Unlike some forms of car racing, riders do not have radios.
green_wave GREEN FLAG (or lights) This is shown to indicate that the track ahead is clear. It may be shown during the pre-race siting lap and also immediately after an incident to indicate that the riders may resume racing.
yellow_steady STEADY YELLOW FLAG (or lights). This is shown to warn a rider that they are approaching an incident. Overtaking is forbidden and riders must be prepared to slow down and stop
yellow_waveWAVED YELLOW FLAG. This is shown at the location of the incident. Overtaking is forbidden, riders must be prepared to slow down
red_steady RED FLAG. This means that the race or session is stopped. Riders must slow down and proceed back to the pits at the earliest opportunity. Red flags may be medical (i.e race is stopped due to an incident requiring medical intervention) or technical (i.e race stopped due to dangerous conditions on track, torrential rain etc)
black_wave BLACK FLAG. When shown to a specific rider - the rider must come into the pits at the earliest opportunity. Often shown to riders who haven't noticed that their machine has developed a technical problem e.g oil leak, fire. May be shown with a rider number board for clarity.
check_steady CHEQUERED FLAG. Indicates the end of the race or session


Some other flags

striped YELLOW/RED STRIPED FLAG ("slippery flag") Indicates reduced grip levels at this section of the track e.g oil, rain, debris on track

white WHITE FLAG Caution: a slower vehicle is on circuit e.g ambulance