What are they?
Environment Maps are an additional texture that you can make use of to help control how the environment lighting/reflections affecct your models and textures in-game.
Environment Maps are a grey-scale version of your texture (diffuse) with the areas you don't want affected by light masked out using pure black while the other areas are varying shades of grey.
What do they look like?
Here is an example of what they look like:
As you can see this is a grey-scale texture, everything I want to be affected by the environment are varying shades of grey, while the parts I don't want to be affected (Like the fabric) has been blacked out.
How do I make them?
Make a copy of your diffuse texture, open it up in your 2D application of choice and make it grey-scale. Then black out the areas you don't want to shine, always a good idea to make sure the background of your texture is also black.
Then take the dodge tool and lighten up areas like the edge of the blade for example for additional detail. Other areas like the edges of the guard or pommel are good places to lighten up a bit more so they shine more.
Save it with a suffix of _m and the DXT1 compression.
How do I use them?
Open up your model in Nifskope, expand the tree branches and find the BSShaderTextureSet node, look at the block details pane and add the environment map into the sixth texture slot.