What is it?
And... it's free and open source: OpenUI5 is licensed under the Apache License, Version 2.0 - see LICENSE.txt
Go to the download page and get the complete UI5 runtime and the UI5 SDK containing the documentation and many example apps as well as the complete runtime.
bower install openui5/packaged-sap.ui.core
Try the Hello World, read the Developer Guide and refer to the API Reference. Use App Templates as a foundation for your developments (available in SAP Web IDE or here on GitHub). Check out the SCN Forum and stackoverflow (use the tag "sapui5") to discuss code-related problems and questions.
You can get the sources and build UI5 on your own, please check the documentation for the UI5 development setup. You might then want to understand how control libraries are structured and controls are developed. Maybe there's a bug you could analyze and fix?