Dear mingming, you can find a lot of information on secant varieties in Harris's book "Algebraic Geometry, A First Course "(Springer GTM 133), essentially presented as a set of thoughtfully conceived exercises. The ultimate reference on the subject is ZAK's monograph
For those who don't know the concept yet let me briefly outline its basic idea. Given a $d$-dimensional variety $X$ in $\mathbb P^n$, take all the chords joining two points ( maybe not distinct: add tangents) of $X$ and consider the union $Sec(X)$ of these chords.This variety has dimension at most $2d+1$ and generically you have equality.This allows for many nice very geometric constructions, for example by projecting from a point outside $Sec(X)$ ( or from a linear subspace disjoint from $X$) . You can easily show this way that every projective smooth variety of dimension $d$ embeds in $ \mathbb P^{2d+1}$ or that every projective variety is birational to a hypersurface in projective space.
Edit Let me emphasize that the basic technique is very easy. For example, a secant line is determined by two points of $X$, so in the grassmannian of lines of $\mathbb P^n$ you get a $2d$-dimensional variety parametrizing the chords. Since the lines have dimension 1, you get the dimension $2d+1$ mentioned above for the secant variety. This looks very sloppy but the amazing and pleasant surprise is that a rigorous proof is pretty close to this sketch: cf. Harris's mentioned book, Proposition 11.24, page 144.