Sage is a mathematical software system, and this tag is intended for questions involving this software in a substantive way. This tag should hardly ever be the only tag of a question; typically there should be additional tags to indicate the mathematical content of the question. Please note that questions that are purely support-questions on Sage are not a good fit for this site.
Sage is built out of nearly 100 open-source packages and features a unified interface. It combines these software packages and seamlessly integrates their functionality into a common experience. Sage can be used to study elementary and advanced, pure and applied mathematics. This includes a huge range of mathematics, including basic algebra, calculus, elementary to very advanced number theory, cryptography, numerical computation, commutative algebra, group theory, combinatorics, graph theory, exact linear algebra and much more.
Note also that Ask Sage, SageMath's questions-and-answers site, and sage-support, the user support mailing list, are very active and that questions asked there typically get answered faster than on the StackExchange network.