Basically, the world, like all things except God, has an end, but it will be a long time before it happens. In the meantime, Paul warns that people should not take the idea of the end of the world as an excuse to behave poorly. Rather we should use that idea to better our lives.
We need to realize that there is only one thing that we can truly depend on, and that is our God. Nothing else lasts. The best-built wall will crumble to time. The people we count on will not live forever. Even great works of art last but a minute on the scale of the universal clock. This should not be reason to mourn, but to better use the time that we have.