A buildpack is an open-source script that compiles apps as container images for a given programming language. Buildpacks provide a canonical way for containers to be created for all programming languages that need a stable and secure runtime and access to the right package managers to handle dependencies.
Most of the buildpacks used in App Platform are based on the Cloud Native Buildpacks, many of which were open-sourced by Heroku. For this reason, it can be a good idea to look at the Heroku buildpack documentation for your coding language if your app is having issues at build time. However, a number of buildpacks used in App Platform were built in-house at DigitalOcean.