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 $\LaTeX$ code for \(\pm\) is
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.
for other uses of this symbol.