A brain dump is exactly what it sounds like: you write down everything currently occupying your head — the errand, the email you owe someone, the vague worry about a bill, the good idea you'll forget by lunch — without stopping to organise any of it. The trick is doing it in one go, because trying to sort and remember at the same time means you do both badly (you'll lose the good idea while you're still deciding which folder the errand belongs in).
That's what the brain dump button does here: one tap, and everything you type lands straight in Someday, unsorted, just captured. No due date to pick, no priority to assign, nothing to decide yet — that comes after, when your head isn't full and you can actually think about what matters. Once it's all out, move the real ones into Today or Upcoming and leave the rest exactly where it landed. Just try it next time your head feels loud — empty it into the tool before you try to make sense of any of it.