Yelling is what player do to project their chat over a further distance, this means that more people can hear you. You can only yell in the "Default" chat window. If you want to yell you simply click the button in the bottom left corner which is a small microphone symbol. If you click it twice you then whisper your chat. You can also yell by writing "#y" before your message. Yelling is mostly used by people that are buying or selling items, also people yell if there is a PK around or if they are being attacked and are requesting assistance.