The symbols displayed for each boost refers to what stat increase the boost includes. These are as follows:

Overtake - Increase Overtaking 
Block - Increase Blocking 
Grip - Increase Grip
Speed - Increase Top Speed
Corners - Increase Cornering Speed
Wetness - Increase Grip during Wet Weather
Tyre use - Decrease tyre wear
Fuel use - Decrease fuel consumption
Pit Stop Time - Decrease average Pit Stop Time
Reliability - Increase Reliability
Consistency - Increase Consistency
Acceleration - Increase Acceleration