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 x2−5x+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 limx→somethingf(x)=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!