The Polkadot Software Development Kit (SDK) is like a toolbox for creating new things on the Polkadot network without starting from scratch. It’s a set of tools, instructions, and pre-made parts that developers use to build custom blockchains or applications that connect to Polkadot’s network.
If one aims to develop a dApp (Decentralized App) or uApp (Unstoppable App), the Polkadot ecosystem contains various SDKs to tap into the relay chain and parachains. There are several languages already supported – see the tooling page for a detailed overview of different SDKs and libraries that are available.