If $x$ is close to (but not equal) to $a$, then $x$ is either
slightly greater or slightly less than $a$. We can explore these cases
separately. The statement $$\lim_{x \to a^+} f(x) = \ell$$ means
"whenever $x$ is slightly greater than $a$, then $f(x)$ is close to
$\ell$'', while $$\lim_{x \to a^-} f(x) = \ell$$ means "whenever $x$ is
slightly less than $a$, $f(x)$ is close to $\ell$."