The Selection Brush tool makes selections two ways: you can paint over the area you want to select in Selection mode, or you can paint over areas you don’t want to select using a semiopaque overlay in Mask mode.
You can first make a rough selection with a marquee tool, Quick Selection Brush, or other selection tool, and then fine-tune your selection with the Selection Brush tool. You can add to the selection using the Selection Brush tool in Selection mode, or subtract from it in Mask mode.