It is the speed guidance during the Take Off and Go Around phases.
Commanded speed at TO is:
2 engines: V2 + 10
1 engine: V2 or speed existing at time of failure (but not less than V2)
The V2 is always that manually entered in the MCDU. SRS is only armed with this condition and flap handle not at 0.
It is engaged by setting TOGA or FLEX (provided a flex temp has been entered in the MCDU).
At go around: speed at time of go around, but not less than Vapp (Vapp=Vref+5)
SRS maintains a minimum ROC of 120fpm.
SRS guidance ceases at the accel alt (at which time the aircraft will automatically accelerate to the climb speed) or engagement of another vertical mode)
F - minimum speed at which the flaps may be retracted at takeoff. In approach, used as a speed target when the aircraft is in CONF 2 or CONF 3.
Represented by "F" on the PFD speed scale. Equal to about 1.18 Vs to 1.22 Vs of CONF 1 + F.
S - minimum speed at which the slats may be retracted at takeoff. In approach, used as a speed target when the aircraft is in CONF 1.
Represented by "S" on the PFD speed scale. Equal to about 1.22 Vs to 1.25 Vs of clean configuration.
O - Green Dot Speed.
Engine out operating speed in clean configuration. (Best lift to drag ratio speed).
Corresponds also to the final takeoff speed.
Represented by a green dot on the PFD airspeed scale
Also a great resource from FBW them self