A buildpack is a sets of scripts that examines an application's source code to determine how to build and run the application. A buildpack will detect the app's language or framework, retrieve dependencies, compile code or otherwise generate assets needed for running the application.
Uffizzi uses buildpacks to automatically detect and build a container image for your application that can be deployed to the cloud. Different buildpacks exist for different languages and frameworks, so Uffizzi sequentially tests these buildpacks against your application's source code, looking for source files that are common to those languages and frameworks. For example, the Uffizzi Node.js buildpack looks for package.json, while the Uffizzi Ruby buildpack looks for a Gemfile.
You can see a list of languages and frameworks currently supported by Uffizzi's automated detection and build process here: Supported Languages & Frameworks