What is a variety? It is the set of common zeroes for a set of polynomials. For example, for the set of polynomials , the variety is .
Now what is a projective variety? Simply put, it is the common set of zeroes of polynomials in which a one-dimensional subspace is effectively considered one point. Hence, for the variety to be well-defined, if one point of a one-dimensional subspace satisies the variety, every point of the one-dimensional subspace has to satisfy that variety. Confused?
Take the polynomial . The point satisfies this polynomial. Now note that the points also satisfy this polynomial for every . Hence this is a projective variety. Now take . Here satisfies the polynomial for only . Hence, this is not a projective variety.
But why? Why would you want to consider a whole line as one point? When you watch the world from your little nest, every line running along your ine of sight becomes a point. Hence, athough it may be a line in “reality” (whatever this means), for you it is a point. This is the origin of projective geometry, although things have gotten sightly complicated since then.