So long as both parties consent and play safely, most things in BDSM aren’t “bad”. But from the outside a lot of them aren’t easily understood.

In BDSM, a “brat” is a submissive who acts like a bratty child. They’re sassy, they often talk back to their Master. They constantly want attention, they frequently disobey. If you’re not in scene I can imagine you’re thinking “who would want such a relationship?” But there are many Masters who think a passive, totally obedient sub is boring. They want someone with a bit of sparkle and fire. Her feisty spirit energizes his. They’ll seek out a brat to keep things lively.
However, it’s possible for brats to take things too far. I remember being in a club in New York. Everything was going great, everyone was having a good time. Then suddenly a sound carried above the crowd – the unmistakable sharp, sudden **CRACK** of an open-hand slap at strength. Even in a BDSM club, it’s not a sound we heard all that often.
The room went quiet and every person turned to see what was going on. The crowd was savvy enough to know someone had just crossed a line, but who? It turned out to be a pretty little brat with long dark hair and gorgeous tits. She had gone a bit too far with her Master and he simply slapped her across the face, once, hard. By the time I arrived she had collapsed in a weeping puddle at his feet and was begging forgiveness. He ignored her for a time then let her sit on his lap until I lost track of them.
As you can see, brats are pretty high maintenance, but they’re rarely boring. A brat will always be submissive but will never be broken. In theory she can be continually stalked without ever being conquered, which is why they’re so popular.