Schubert calculus is a branch of mathematics that studies the intersection theory of Schubert varieties in projective spaces. It provides tools to count the number of geometric objects, such as lines or planes, that meet certain conditions within a given space. The main focus of Schubert calculus is to understand how these varieties intersect and to compute their dimensions and properties. It uses techniques from algebraic geometry and combinatorics, often involving Young tableaux and cohomology to derive results about the configurations of these geometric objects.