Scilab is a high-level programming language, oriented toward numerical computation, that allows you to perform all sorts of mathematical operations and combine them to create sophisticated programs.
Scilab's main operations include: linear, non-linear, and differential equation systems; derivation, integration and interpolation; linear, non-linear, and quadratic optimization; 2D and 3D graphics, animations, matrices, and more.
Of course, when dealing with a programming language it's important to understand its syntax and main commands. You can find an introductory PDF document to help you get started with Scilab here.
