A document that details exactly what a system must do: functions, roles, data and constraints, serving as the reference for building and acceptance.
SRS stands for software requirements specification. It is the document that moves a system from an idea in its owner's head to a text you can build from and hold people to. It lists the functions one by one, who uses each, the data that is stored, rules such as 'a posted invoice cannot be deleted', and non-functional requirements such as language, security and performance.
Without it, every project starts with enthusiasm and ends with 'this is not what I asked for'. Government bodies and large companies require one before any contract, and small projects need a lighter version for the same reason. Writing it used to take a business analyst weeks. In Fanann, the Product Owner Noura asks the questions that settle scope, then writes the document and the task list, revises any part you select and hands it to the build stage. Practical tip: read the out-of-scope section before anything else. What you agreed not to build protects you from a ballooning project more than any other clause.