We say that a number $x$ is within an absolute error of $y$ to a correct solution if it
is within $y$ of the correct solution $c$, meaning that $|x - c| \leq y$.
For example, if our correct solution is 6.157892, then
$x$ is within an error of 0.001 (or three decimal places) for any $x$ such that
$|x-6.157892| \leq 0.001$; equivalently, $6.156892 < x < 6.158892$.
Rosalind uses a default absolute error of 0.001 for counting solutions correct.