Participating in the Ethereum Blockchain Network requires a connection to an Ethereum Node. With Infura you can get API access to a Node without running one yourself.
Their generous free plan should be more than enough to get you started with your Smart Contract and dApp projects. Once outgrown, upgrading to the next tier is a matter of a few clicks.
Protip: Infura allows you to connect to their network via WebSockets (wss://) which can help you save bandwidth and API credits with read- and write heavy operations.
- ConsenSys