User interface

Here is a screenshot of the default Irssi user interface, with all the major visible things annotated.

topic: channels often configure a topic with more information about the channel. Type /t (short for /topic) to read the full topic
window number: each window (“Tab”) in Irssi has a number. You can go to a window by number by typing /<number> (e.g. /2). To change the number, see window number changing
channel mode: the channel mode is a protocol-specific list of letters and arguments to configure the channel behaviour. Each network may have its own custom channel modes. See IRC Client Protocol – Channel Modes for some common modes and your network website for network-specific modes. Note, the status bar does not show list modes (like bans) nor channel membership prefixes (like channel operator).
channel name: the name of the channel that you are joined to
network tag: the Irssi-internal name tag assigned to your current connection/network
user mode: the user mode is a protocol-specific global list of letters that reflect properties of your nick/connection. Each network may have its own custom nick modes. See IRC Client Protocol – User Modes for some common modes. You can use /set usermode in Irssi to configure your desired default user mode or /umode to change your current user mode (where permissible; not all mode letters can be changed by yourself).
your nick: your current nick name on the server. You can set your default nick with /set nick in Irssi and change your current nick with /nick.
clock: shows the time on the computer where Irssi is running
active window indicator: shows where any text you enter will be sent to
activity indicator: this list will tell you about new messages in windows that are currently not visible. It reads Act: 1,2,3,5,7,.... Each number that lights up indicates that there are unread messages in that window. The colour of the number depends on the type of message – white/black: regular text, cyan: status messages, magenta: hilights or private messages
root status bar: the root status bar type is a status line that is either at the very top or at the very bottom of your screen. if the root status bar contains window-specific information, it will always refer to the currently active window
window status bar: the window status bar type is a status line that is either at the top or bottom of a split window. if you create a window type status bar, it will be replicated on all inactive split windows. window status bars can show information specific to the window that they belong to. See Help: statusbar for help on configuring the status bar.
split window: multiple windows can be shown on the Irssi screen at the same time. See Help: window for help on managing windows.