What are the iron bars and threaded rods sticking up out of my backyard?
It’s a modified type of “sheet piling” used for earth retention/ engineering. 

I’d guess there is (was) some slope at your site that the previous home owner wanted to control. I’d guess that they started a project and then abandoned it because it looks like some preliminary site work. 

If you remove it, be careful as it could loosen some surrounding areas that could start moving. 

The threaded rods are obviously set for future connections, building, equipment, shoring, retaining, etc. 

It will be difficult to cut off, based on its size and shape. (The curved connections between the flanges and web indicate it’s structural steel...probably A36 steel, minimum.)