This is the first of the Six Pillars of Calculus.
In most previous math classes, we have learned how
to get exact answers. If we want to solve $x^2 - 5x + 6 =0$, the answer isn't
"close to 1.99" or "close to 3.01". The quadratic formula tells
us: "$x$ is exactly 2 or exactly 3".
In calculus, we have problems where we can't get an exact answer
directly. Instead, we find an approximate answer, then a better
answer, then an even better answer. The exact answer is the limit of these
approximations.
The general form of a limit statement is $$\lim_{x \to
\hbox{something}} f(x) = \hbox{something else},$$ which means
"whenever $x$ does something, $f(x)$ does something else".
This is the most important idea in all of calculus. Learn it well!