A backdash (input b~n~b) moves backwards quickly, creating space between the opponent to make it more likely their attacks will whiff. The full duration of a backdash maintains standing guard, and the backdash can be cancelled at any time by any other move, making it a low risk option.
It's not possible to do more than one backdash in a row by pressing b,b repeatedly. The first backdash has to finish completely before another can be done, which takes a long time. Instead, to create more space than a single backdash does, you have to do a backdash cancel.
Backdashes can't be fully buffered, so doing a backdash out of a blockstun or hitstun requires good timing. The first b can be buffered, but the second one must be input after you've recovered. From crouch, an extra frame is needed (see crouch cancel). Getting this timing right can make a huge difference in a lot of pressure situations.
Because they can't be buffered, trying to backdash at a heavy frame disadvantage can be risky without precise input. If for example you try and backdash at -9, but you mistime your backdash by a single frame, you can get hit by i10 attacks. This is why sometimes people do jab strings even at +9—they might catch you lacking.
Variation between characters
Different characters cover more space than others with their backdash. The character with the best backdash in terms of distance moved is Zafina, and the worst is Gigas. However, the exact amount of evasiveness a backdash grants also depends on the position of hurtboxes, so there are some mixups out of pressure that Gigas can avoid with a backdash but others can't.