Basis reduction and continued fractions

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While reading several articles about lattice basis reduction I am left with a few questions.

For one, I came across this piece of text

Let $\alpha$ and $\beta \in \mathbb{R}$. Then there are two almost the same ways to compute small values for $\alpha x + \beta y$ with not too large $x,y \in \mathbb{Z}$.

1) applying the continued fraction algorithm

2) Applying the lattice basis reduction algorithm to the lattice generated by the columns of the matrix

\begin{pmatrix} 1 & 0 \\ C\alpha & C\beta \end{pmatrix} for $C$ large enough.

Why are those (for me different algorithms) in the above sense the same?

And also, where is the $C$ coming from? When is it large enough? It obviously depends on something...

All hints, examples or explanations are very much welcome.