To add it, go to add a new widget and look for Traffic. When you add it, you will have to give the widget a name and then give it an address to navigate to. The widget will then be placed on your homescreen and will look something like the picture here. It will show you a red, yellow, or green light based on current traffic conditions and estimate the number of minutes it will take to get from your current location to the destination in the widget.
You can add multiple traffic widgets to your homescreen to the various places you travel most. One for home, one for work, etc. Then before you leave for that place, just tap the widget and you get a quick heads-up about what the traffic conditions are along your route. The time estimation isn't perfect, of course, as routes can vary greatly depending on traffic lights and other factors, but it's still a great way to get a quick glance at what might be along your route. If nothing else, the light showing red when you don't expect it might make you pause to look up what's going on.
.