When you say "hi", "hello", or "ashari" (for elf NPCs) to an NPC the conversation will be moved to this channel. Here players speak in blue and NPCs speak in cyan, some key words are highlighted. Conversations will be seen in the channel and in the same colors above the player and NPCs heads. You can type in the NPCs channels even if you are not talking to an NPC. Clear the messages from the console window by repeatedly pressing Ctrl+W (Tibia 10) or Alt+W (Tibia 11's default hotkey).
The NPCs Channel was added with the Summer Update 2008 as a solution for the waiting lines at banks and other miscellaneous NPCs. With the channel, no longer will a player be muted for boating to other cities too fast, nor will they be muted for buying/selling items with the implemented trade dialog box.