We've designed Ergo with long-term economic sustainability in mind, and storage rent is one of the ways we're ensuring miners stay profitable well into the future. This can be thought of as 'on-chain garbage collection' that reduces the problem of blockchain bloat – and even makes it profitable.
Storage rent is a nominal fee charged on unspent output after four years, with a price per byte defined by the storage rent subprotocol. This works out to about 0.14 ERG per four years for your average box.
You can read more details here