Movement is mainly used to cause or prevent moves from whiffing. There are three main directions one can move: backwards (which creates whiffs), forwards (which prevents them), and sideways (which can do both). Movement can also be used to get a more favorable wall position.
The key to good movement is less about the movement itself and more about what is done after it. If one creates many whiffs but never punishes them, then of what use was the movement?
Forwards movement is faster than backwards movement, so it's always possible to force pressure by dashing into the opponent. Moving forwards also gives access to dash and run moves, with the strongest of these being the armored shoulder and running slide.
Backwards movement—unlike sideways or forwards movement—can be done while guarding, making it much safer.
A dash block is a technique used to safely start pressure. If one does a dash into attack to start pressure, this risks losing hard to many keepout moves. With a dash block you instead block the keepout move and use the frame advantage for pressure.
A step block is a technique used to create whiffs with sideways movement without taking on the risk of not being able to guard while sidestepping.
Most characters don't have many fast moves to worry about, and most of them are linear. A step block done on a timing that blocks exactly when the opponent's fastest scary move would connect is able to safely block most attacks while creating the possibility of a big reward off a whiff punish. This is in most cases roughly 12 frames of sidestepping.
There's no real trick to it other than timing. Just press b during a sidestep at the right time.
Backdashing is used to create space between you and the opponent while still blocking mids and highs. This is useful to escape their pressure, and to make them whiff so you can get a whiff punish. It's not flashy, but this is extremely safe and often very rewarding. The average whiff punish outdamages the average low, and backdashing doesn't risk being counter hit or punished. Additionally, an opponent chasing after you is easier to hit, and an opponent who is careful with their spacing to avoid a whiff punish has to attack you less.
That said, it's important not to backdash yourself to the wall. With your back to the wall, your opponent can control their spacing much more than you can control yours. And if you backdash relentlessly, you'll never bait any whiffs.
Also worth mentioning is that different characters have different backdash lengths.