The first number is the tire width, measured at the widest part of the sidewalls, in mm. So a 265 tire is 265 mm wide and a 285 tire is 285 mm wide (or 20 mm wider than the 265) at their widest points.
The second number is the "aspect ratio" or the height of the sidewall compared to its width. In your Duratrac examples, the 70 means that the sidewall height is 70% of its width, so a 70 in a 285 (199.5mm) will be higher than a 70 in a narrower 265 (185.5 mm) tire (70% of 285 is greater than 70% of 265).
As for what you can fit on your Jeep, there are loads of threads in this Forum going into great detail about that. Different people have different opinions and different experiences, but it seems that a 285 should fit your JKU, but will possibly rub at full lock and/or full flex (assuming no spacers or lift).
Not to confuse you even more, but another consideration you have to look at is how wide of a tire can you mount on your rims.
Also, just my opinion, but a 265 may look a bit small on a JKU. I am running 285/70/17, which I think are perfect for my needs, but which many on this Forum think is too small. There are lots of picture threads on this Forum that can show you what a particular tire size tire will look like on a Jeep. If you can't find one, post a request looking for it.