Operators (Op), or more specifically Channel Operators, are specific user mode within a channel. That mode is +o, in the Main client of Mibbit they are represented by a . In general, they act as channel administrators.
The first user to join a non-registered channel will be made an Operator. Operators can also be promoted by other operators. They have access to tools to manage a channel, such as kick, ban, topic, and other channel modes.
Channel Operators are not IRCOps.