ok I got 1 open edge .... how do I fix it? explain in detail

I'm still not getting the target weld thing? more detail?

Let's say you have a box, with all it's faces facing inwards like the most basic map you can make in halo. it's sealed and has no open edges, it will work.

Let's say you take out one of those faces, the box is no longer sealed, and it has 4 edges (were the face was) that are open.

To fix this, you have to seal the map again, or close those open edges by either making a face between those 4 open edges or welding them together. In the box example if you welded all 4 open edges you'd end up with a pyramid, that complied with the sealed world rule and would work in halo.

ok first I only have one open edge second it's in the air. so now how do i fix it. no one is really givving me help that I understand

to weld, select all the verticies you wish to weld, and specify a distance. This distance is the radius each vertex checks around to see if any other selected verticies are there to weld with. Welding means they combine into one vertex.