Thresholding is different from grayscale. A grayscale picture has many shades between black and white. A threshold result assigns each visible pixel to one of those two extremes.
Black and White Threshold compares calculated luminance with your cutoff. Pixels at or above that value become white; lower values become black, while existing alpha is retained.
Try a cutoff you can understand
At a cutoff of 128, a pure red pixel becomes black because its weighted luminance is below the midpoint. Pure white remains white and pure black remains black.
The calculation uses weighted RGB channels, so a saturated colour's brightness is not determined by the largest channel alone. Green contributes more to luminance than blue in this operation.
Changing the cutoff can reveal or erase thin strokes. A light grey pen mark may disappear into white when the cutoff is too low for the surrounding scan, or a noisy background may turn black under another choice.
Inspect the information that matters
For a signature, examine the faintest parts of the strokes. For a document, inspect small text and punctuation. A cleaner looking page is not useful if it loses the marks needed to read it.
This is a global threshold, not adaptive document cleanup or OCR. Uneven lighting can make one cutoff unsuitable for both sides of a photographed page.
The dimensions stay unchanged. Use resizing separately if the receiving form also requires a smaller canvas.
Keep the original colour or grayscale image until the threshold copy has been reviewed. Black and white conversion discards tonal information, so returning to a downloaded threshold result cannot recover the shades removed by the operation.