Basically it makes the EU a bit more like a real country, and a bit less useless. Its designed to replace all the years of trade agreements and treaties into one single document, and add some powers to things like the European Parliament and the Presidency, and give it a foreign minister.
In actuality, I doubt much will change. However, watching the Tories lose their minds over it has been amusing. The Tories secretly like the EU, because, well, its a free trade organisation which benefits their backers and they approve of that. But they are also willing to use nationalist sentiment against the EU to gain support for their party, and then when in power, wring their hands about how "there is no clearly defined process for leaving, so we will have to stay in for now".
Only, the treaty has provisions for how countries can easily and freely leave the EU, so the Tories are on a mad dash around Europe to try and stop other nations voting for it.