"Multiple panes" refers to having several separate panels or sections within a larger window or screen. It allows you to view and work with different content simultaneously, side by side. Full definition
With multiple panes open side by side, searching the web and writing out notes are simple and won't drastically drain battery life. (heavy.com)
The cabin maintained pressurization because each window is made of multiple panes, Southwest said, adding that no emergency landing was requested. (cnbc.com)
On the iPhone 6 Plus only, Apple allows apps to use multiple panes, something Samsung apps like the Galaxy Note's email client have done for years. (tomsguide.com)