The Blender interface is entirely customizable. You can create as many panes as you want, arrange them however you want, and any pane can be for any context/view. This is great... once you know what's going on ;-)
The default layout is very similar to what you're showing, so I'm guessing that you're wanting the 3D Viewport (where you edit meshes) to be back in the largest pane, where it usually is. You can do this by choosing the "3D View" from the drop-down menu on the bottom left of that pane.
The way you got there is probably by pressing F11, which brings up the render result (which is what you're looking at) without starting a new render. Rendering will also do this. Either pressing F12 or pressing the Render button automatically replaces one of your panes (I'm not quite sure how it chooses which one) with the Image Editor, which is also where you view the final rendered image. Typically, pressing Esc will return the Image Editor view to whatever it was before. Sometimes that doesn't work, but you can always* manually choose the 3D View again.
*The one case you can't change the view is if a pane is maximized. Pressing ctrlspace or ctrlup will toggle the pane your mouse cursor is in from its normal size to taking over the whole screen. This is a good way to maximize screen real estate when you're focused on a particular task, but it can be disconcerting if you don't know what's going on.
Incidentally, the 3D View is also what you have for the bottom right pane, which is usually the "Properties" view. If you change that back to the Properties view, it will be more-or-less back to the default layout.