It's just virtually "mining" for now. From the pi faq :
Do I need to leave the app open to mine? Does the app drain my battery or data?
You do not need to leave the app open to mine. Pi does not affect your phone’s performance, drain your battery, or use your network data. Once you hit the lightning button, you can even close the app and you will continue to mine Pi. Read less ...
So how can you mine a cryptocurrency without draining your battery or data? Instead of burning energy as proof of work cryptocurrencies like Bitcoin do, Pi secures its ledger when members vouch for each other as trustworthy. This forms a network of interlocking “security circles” that determines who can execute transactions. This novel approach allows crypto mining on your phone by leveraging your existing social connections, with no financial cost, no battery drain and a light footprint on the planet. Read the technical section in our White paper for more accurate and detailed explanation.