Boost Speed is the speed that a ship travels when the boost function is activated (Default "Tab" or "B" on Xbox). When activating boost the ship's thrusters are pushed past their normal capabilities which provides extra speed and maneuverability. Boost can only be activated in normal space when there is enough energy in the ships Engine capacitor and generates heat upon activation. Upgrading equipment can allow for more frequent boosts and/or a higher boost speed depending on the module that was upgraded. Currently boost speeds vary from ship to ship and range from 200 m/s to 460 m/s. It is unwise to boost out a star port.
Boosting is very useful in combat and when used correctly can provide a significant edge. Activating a boost and quickly turning with Flight Assist Off can allow a pilot to change the direction they're facing. Spamming boost can allow a player to move faster than usual and possibly outrun attackers or catch up with prey - this tactic works better with some ships more than others. For example while the Viper has the fastest normal speed; the Cobra can move faster when constantly activating boost.
The automatic deceleration after the boost is managed by the on-board Flight Assist.