Polkadot will enable a completely decentralized web where users are in control. Polkadot is built to connect private and consortium chains, public and permissionless networks, oracles, and future technologies that are yet to be created. Polkadot facilitates an internet where independent blockchains can exchange information and transactions in a trustless way via the Polkadot relay chain.
Polkadot has a very informative wiki-page. We do recommend all beginners to start from learning it: Learn Staking
The best instructions on how to stake Polkadot (DOT) also can be found on their wiki-page.
There are to ways to earn Polkadot (DOT) by staking. You can be eather a validator or a nominatore.
While the experience of a nominator is similar to "set it and forget it", in reality there are many reasons to keep an eye on one's validators and keep optimizing the nominations for best returns and reduced risk.
Polkadot is a sharded protocol that enables blockchain networks to operate together seamlessly.