What is SBML?
The short answer is this: the Systems Biology Markup Language (SBML) is a machine-readable exchange format for computational models of biological processes. Its strength is in representating phenomena at the scale of biochemical reactions, but it is not limited to that. By supporting SBML as an input and output format, different software tools can operate on the same representation of a model, removing chances for errors in translation and assuring a common starting point for analyses and simulations. A slightly longer (but still relatively short) answer can be found in the separate Basic Introduction to SBML.