I would try to explain it myself, but in truth when I came across the walls I never really thought twice of them. I did however Google "Half Life 2 walls" with a successful result.
http://half-life.wikia.com/wiki/Combine_Smart_Barrier
This should explain what they are far better than anything I could come up with.
The Combine Smart Barrier, also known as the Combine Mobile Wall, is a large mechanized wall that uses powerful drive systems in tandem with heavy metal blocks to slowly expand outward and destroy existing structures in the way.