After a channel is created, the funds then must exist on the correct 'side' of the channel for a payment to be routed correctly between two connected nodes. Back to an example - Alice and Bob are friends and go out for food and drinks on a regular basis. Rather then Venmo each other, they decide to open a lightning channel with $100. If Alice opens the channel to Bob, she will have to commit $100 (in BTC), which is then locked up and seen in an on chain transaction (on the Bitcoin blockchain). Since Alice deposited the funds in the channel, all of the 'liquidity' (money) is on her side. Bob does not have any money, and cannot yet use the channel to send money to Alice. So Alice will have to metaphorically 'buy the first round' and send the first payment from the committed funds to Bob (any amount less than the channel capacity). Once that money has been sent, Bob can then send up to that amount back to Alice, and they can continue in this fashion.