NOTE: If you are developer building apps on top of ACA-Py and Indy, you DO NOT need to know the nuts and bolts of Indy to build applications. You need to know about verifiable credentials and the concepts of self-sovereign identity. But as an app developer, you don't need to do the Indy getting started pieces. ACA-Py takes care of those details for you. The introduction linked here should be sufficient.
If you are new to Indy and verifiable credentials and want to learn the core concepts, this link provides a solid foundation into the goals and purpose of Indy including verifiable credentials, DIDs, decentralized/self-sovereign identity, the Sovrin Foundation and more. The document is the content of the Indy chapter of the Hyperledger edX Blockchain for Business course (which you could also go through).
Feel free to do the demo that is referenced in the material, but we recommend that you not dig into that codebase. It's pretty old now - year old! We've got much more relevant examples later in this guide.
As well, don't use the guidance in the course to dive into the content about "Getting Started" with Indy. Come back here as this content is far more relevant to the current state of Indy and ACA-Py.
Indy provides an implementation of the basic functions required to implement a network for self-sovereign identity (SSI) - a ledger, client SDKs for interacting with the ledger, DIDs, and capabilities for issuing, holding and proving verifiable credentials.
Back to the ACA-Py Developer - Getting Started Guide.