Basics of Movement[]
Space is big.
There have been a number of more qualified and better written descriptions about how big space is before, so let's not go too much into that now. Suffice to say, space is just big enough to fit all the nothing inside of it.
That whole lotta nothing means that there's no air friction, and not air friction means no slowing down. Newton's first law, folks - An object in motion stays in motion unless acted upon by an outside force. In this case, the outside forces that matter are your AP, your enemies, and the wall.
Take this scene, for example. A Quicksilver is charging at a stationary Albatross near some asteroids. Both of them are not moving.
As a light Frame, the Quicksilver has 4 Action Points to spare. You may change your momentum by two for each AP spent on movement - for a total of 8 for a light frame. If, for his action, the Quicksilver were to accelerate 6 to the left (while the albatross does nothing - not likely in an actual conflict, but you never know), the scene would look at like this at the end of the turn.
As you can see, the Quicksilver has moved six to the left. However, now there's something new on the map - A faded version of the 'silver's token with a yellow outline. What's this?
This is your velocity indicator. It shows you where you'll move at the end of the turn - since his momentum is conserved, this is where the Quicksilver will move to if he does nothing this turn and instead spends his time doing something else, such as shooting. However, suppose the quicksilver is realizing he'd prefer not to get that close to the enemy, and decides to retreat - spending two AP to go right four. This results in this:
Now that wasn't what he intended at all! What happened here?
The basic idea that the Quicksilver failed to remember is that when accelerating, you need to think a turn ahead - When he accelerated four to the right, he didn't move himself four to the right. Instead, he lowered his leftward momentum by four - which still left two going left. As such, at the end of the turn he moved two squares to the left, and will move a further two squares next turn unless he does something to change it.
TL;DR: When accelerating, think of how your yellow-outlined velocity indicator will change, not your actual token. If you move two to the right, it is your indicator that will move two to the right. Your previous acceleration is maintained.
Movement and Combat[]
So how's attacking work, you may ask? That is, after all, what Frames are for.
You can essentially think of the game as working in two distinct stages - a "Movement" stage and an "Action" stage. However, unlike most games that use a similar system, the "Movement" stage comes second. When considering ranges of enemies and possible targets, you should think in terms of your token - not your velocity indicator. After all units take their actions, movement will take place. Naturally, though, there are a few exceptions to this mechanic - namely, Prepared Attacks.
There are two varieties of prepared attacks - those using a gun or ranged weapon and those using a melee weapon. The latter is often known by its other name of 'charging'. Both work on similar principles: With a prepared attack you use your velocity indicator's location to determine range and possible targets, not your token. This, however, may only be done once per turn, and for ranged weapons requires you to spend an extra AP to 'aim' beforehand - tracking your target to keep them in your sights. For melee weapons, this is not required- but keeping up with the enemy is not easy to begin with.