i3 is a tiling window manager.

A window manager is a system software that controls the placement and apperarance of windows within a windowing system in a graphical user interface(GUI). It can be part of a desktop environment or be used standalone.

And,

Tiling window manager is a window manager with an organization of the screen into manually non-overlapping frames.

Here's an image from i3wm-docs.
i3.png
Confusing?? Try i3! Give it a shot and u will find out.

Various operations in i3wm are performed using the keybindings and it's fast.One of the greatest feature is multi-monitor support.I was always inspired by the english movies ( especially those hacker movies) where a guy sits in a basement with multiple monitors in front of him. Those scenes were really cool but i always wondered how do they do it! Of course, they might not be using i3wm but that can be done using i3 window manager. Other linux desktop environments also provides the feature of multiple monitors but i think they are not as awesome as of i3wm. I have been using i3wm since some months and i find it really awesome.Especially with it's multiple monitor support. Connect a new monitor and that's it.It provides you with a new empty workspace. Use your keybinding,jump to any workspaces and monitors, do some task, open some windows, check some outputs, copy paste something,use IRC or Browse,presentations. If you are a command line lover you may wanna use xrandr to configure those multiple monitors/displays.There are some GUI applications too like lxrandr and arandr. After connecting the display, use this command to find out the information to configure.

xrandr --query

You may get output like this:

Screen 0: minimum 8 x 8, current 2732 x 768, maximum 32767 x 32767
eDP1 connected 1366x768+1366+0 (normal left inverted right x axis y axis) 340mm x 190mm
   1366x768      60.10*+
   1024x768      60.00
   1024x576      60.00
   960x540       60.00
   800x600       60.32    56.25
   864x486       60.00
   640x480       59.94
   720x405       60.00
   680x384       60.00
   640x360       60.00
DP1 connected 1366x768+0+0 (normal left inverted right x axis y axis) 340mm x 190mm
   1366x768      59.79*+
   1280x720      60.00
   1024x768      60.00
   800x600       60.32
   640x480       59.94
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)

Note the names of the connected ones and their supported/used resolutions.In my case those are eDP1 and DP1. Now if I want to place eDP1 to the right of DP1 then i would run the following command:

xrandr --output DP1 --mode 1366x768 --output eDP1 --mode 1366x768 --right-of DP1

If you want to mirror both of these displays then replace –right-of in above command with –same-as

xrandr --output DP1 --mode 1366x768 --output eDP1 --mode 1366x768 --same-as DP1

GUI applications also can be used to perform these applications easily.

i3 was specifically developed with support for multiple monitors in mind.

So, when connected with multiple monitors, each monitors get's an initial workspaces as i previously mentioned. Monitor 1, monitor 2 and monitor 3 gets workspace 1 , workspace 2 and workspace 3 respectively if they are connected in that order. As,i3wm has special keybindings to switch between workspaces, you are automatically switching between monitors also.That' easy isn't it.

So, one last thing I am going to mention is how to move workspaces to other screen. In you i3 config file generally ~/.config/i3/config

bindsym $mod+n move workspace to output right

So, now i can easily move the workspace to another screen using the $mod+n keybinding as I have placed my monitor to the right.