Software Development Kit - collection of tools, libraries, and documentation for building applications. Like a toolbox with everything needed to build a specific type of project.
Mobile developers use the iOS SDK to build iPhone apps with access to camera, GPS, and other device features.
SDKs are developer toolkits (libraries, tools, docs) rather than a single cloud service. AWS, Azure, Google Cloud, and OCI each publish many SDKs (e.g., for Java, Python, JavaScript, .NET) to help apps authenticate and call their APIs.