To quote the official description:
Scalar is a free, open source authoring and publishing platform that’s designed to make it easy for authors to write long-form, born-digital scholarship online. Scalar enables users to assemble media from multiple sources and juxtapose them with their own writing in a variety of ways, with minimal technical expertise required.
More fundamentally, Scalar is a semantic web authoring tool that brings a considered balance between standardization and structural flexibility to all kinds of material. It includes a built-in reading interface as well as an API that enables Scalar content to be used to drive custom-designed applications. If you’re dealing with small to moderate amounts of structured content and need a lightweight platform that encourages improvisation with your data model, Scalar may be the right solution for you.
Scalar is not primarily my work–Craig Dietrich, Erik Loyer, and others are much more responsible for it than I am–but I did contribute several pieces of code the project. I was primarily responsible for API development and consulted on various security, database, and architecture related issues.