Miners must prove they can store client data. This is called Proof of Replication. Miners then have to submit additional proofs called Proof of Space-Time to show they are fulfilling responsibilities. In return for storing data, miners receive rewards in FIL coins. Note, the Filecoin website warns that storage mining is an advanced process, with notable hardware requirements i.e. GeForce RTX 3090. You can estimate potential earnings by using a mining profitability calculator online. For more information on mining rigs, hardware and other requirements, download the relevant docs from the Filecoin website.