A blockchain oracle is a decentralized data services that connects smart contracts with the outside world primarily (e.g. weather data, Vessel data, stock market data etc.) to feed information in from the world, to the blockchain. Oracles also provide a way for blockchains to see into each other. This is known as interoperability, and is an important next step as well.
Why are blockchain oracles so important?
We need the blockchain world to connect with the real world. We need to get the weather data so we can have decentralized trustless insurance. We need data to use blockchain for one of its most important purposes, smart contracts. We need to get the price of ETH and other cryptocurrencies into a contract so we can have DeFi. Blockchain oracles are a bridge between the two worlds, the real world and the blockchain world.