To bleach is to whiten, or to strip of color. After many summers of use, the sun will bleach your favorite beach towel.
You might bleach your white laundry, or bleach your dark brown hair until it's pale blonde. In both cases, you use bleach, a strong chemical solution that can often be found in laundry rooms and hair salons. The Old English root word is blæcan, "bleach" or "whiten," from a Germanic source it curiously shares with the word black.