IRC commands allow perform specific functions on IRC, such as maintaining control of a channel and the users on it. Many accept or require arguments.
Most IRC clients (IRC scripts) require that the commands to be preceded by a forward slash /, others by /raw and some are sent to bots IRC. These, are treated by the IRC protocol as common messages and not commands. Anything that doesn't start with / is considered a message that you are typing to someone or some channel.