A network is an Ethereum environment. This is where users test, develop, and interact with others.
There are different networks available. The most common are the Ethereum Mainnet, the Ethereum Testnet, and Layer 2 networks.
The Ethereum Mainnet is the base blockchain where on-chain transactions occur. When you send ETH to a friend's wallet on the Ethereum network, you're using the Mainnet. This is also referred to as Layer 1.
Layer 2 networks are where off-chain transactions occur. As the name suggests, Layer 2 networks operate on top of Layer 1. These networks reduce the cost, and time, of transactions. Optimism, and Arbitrum are examples of Layer 2 Networks.