Speed refers to the rate at which an entity such as a ship or laser is able to move across space. The max speed is different for each ship. The time it takes to reach this max speed or acceleration is determined by the agility of the ship.
A ship's speed is defined by the fastest it can go while going forward(left mouse button/forward arrow key). The ship has a maximum speed while doing this. As you may notice while using recoil(see U-Sniper and Shadow X-3) or getting knocked around by a Barracuda, you can exceed this maximum speed. The Friction setting will adjust how quickly you are brought back to still or moving at your maximum speed. Setting the friction at 0 in a Custom Game makes gimmicks like hyper speed possible.
Ship speed and laser speed can be Upgraded with the red colored speed upgrade(marked with number 7) for ship speed and the orange speed upgrade (marked with number 6) for shot speed and knockback.