The Hedera mainnet (short for main network) is where applications are run in production, with transaction fees paid in HBAR. Transactions are submitted to the Hedera mainnet by any application or retail user; they're automatically consensus timestamped and fairly ordered.
Data associated with Hedera's services and stored on-chain can be queried by any Hedera account. Every transaction requires payment in the form of a transaction fee denominated in tinybars (100,000,000 tℏ = 1 ℏ). You can learn more about transaction fees and estimate your application costs here.
If you're looking to test your application (or just experiment), please visit Testnet Access. The Hedera testnet enables developers to prototype and test applications in a simulated mainnet environment that uses test HBAR for paying transaction fees.
Transactions on the Hedera Mainnet are currently throttled. You will receive a "BUSY" response if the number of transactions submitted to the network exceeds the threshold value.