We want to show that $f$ is continuous at a point $x_0$, say. The condition for continuity says that
Given any $\epsilon>0$, we can find a $\delta>0$ so that the following statement is true:
If $ \lvert x - x_0 \rvert < \delta $ then $\lvert f(x)-f(x_0) \rvert < \epsilon $.
We want to prove this from stuff we know about the $f_n$. We know two things: firstly, that they are continuous, and secondly, that they converge uniformly to $f$.
Since $f_n$ converges uniformly to $f$, we can find an $N$ that is independent of $y$ so that
$$\lvert f_n(y) - f(y) \rvert < \epsilon/3 $$
for any $n>N$, and any $y$ in the set. (In particular, this is true of both $y=x$ and $y=x_0$.)
Suppose we have such an $n$, $N+1$ will do, and now use that $f_{N+1}$ is continuous. Hence we can find a $\delta$ so that
$$ \lvert f_{N+1}(x)-f_{N+1}(x_0) \rvert < \epsilon/3 $$
whenever $\lvert x - x_0 \rvert < \delta$.
We now use the triangle inequality:
$$ \lvert f(x)-f(x_0) \rvert \leqslant \lvert f(x)-f_{N+1}(x) \rvert + \lvert f_{N+1}(x)-f_{N+1}(x_0) \rvert + \lvert f_{N+1}(x_0)-f(x_0) \rvert $$
Supposing now that $\lvert x - x_0 \rvert < \delta$, we apply the uniform convergence to the two end terms and the continuity of $f_{N+1}$ to the middle term, and find
$$ \lvert f(x)-f_{N+1}(x) \rvert + \lvert f_{N+1}(x)-f_{N+1}(x_0) \rvert + \lvert f_{N+1}(x_0)-f(x_0) \rvert \leqslant \frac{\epsilon}{3} + \frac{\epsilon}{3} + \frac{\epsilon}{3} = \epsilon, $$
which is precisely what we wanted.
