Defines a range in which a number may lie.
$x = a \pm b$ means $a - b \le x \le a + b$.
In the context of engineering, it is known as a tolerance, arising from the idea that any number within $\pm b$ of $a$ is a within a tolerable range of the desired value.
(The despicable political term zero tolerance was supposedly coined as a reflection of the above engineering definition. It of course takes someone with an actual mathematical or engineering background to understand how poisonously unachievable such an aspiration is.)
The $\LaTeX$ code for \(\pm\) is
See Arithmetic and Algebra for an alternative definition of this symbol.