What ± means
The plus-minus sign ± has two jobs. In algebra it means "plus or minus": if x² = 9, then x = ±3, so x is 3 or −3. The best-known example is the quadratic formula, x = (−b ± √(b² − 4ac)) / 2a, which gives two answers at once. See the square root symbol page for the √ part.
In measurements it marks a range or tolerance. A part made to 10 ± 0.1 mm can be anywhere from 9.9 to 10.1 mm, and a poll with a margin of error of ±3 points may be off by up to 3 points in either direction.
Plus-minus vs minus-plus ∓
The minus-plus sign ∓ appears together with ± to show that the signs flip in step. In cos(a ± b) = cos a cos b ∓ sin a sin b, you take the top signs together (+ on the left, − on the right) or the bottom signs together.
While you are here, note the difference between the true minus sign − and the hyphen - on your keyboard. The minus sign is as wide as + and lines up with it, so formulas look cleaner. Typed text often falls back to +/−, but ± is clearer and shorter.
How to type ±
On Windows, hold Alt and type 241 on the number pad (Alt + 0177 also works). On a Mac, press Option + Shift + =. In Microsoft Word, type 00B1 and press Alt + X. In Google Docs, use Insert > Special characters and search for "plus-minus".
In LaTeX, use \pm for ± and \mp for ∓. In HTML, write ±. On phones, copy ± from this page or add a text replacement. For temperature readings like 20 ± 2 °C you will also need the degree symbol, and the rest of the operators are on the math symbols page.