When you obstruct something, you block it. If you’re gobbling down your pizza, a chunk of crust you didn’t chew so well might obstruct your airway and you'll choke.
Obstruct also means to get in the way so that you hide something from view. That fast food tower being built next to your beach house will obstruct your lovely ocean vistas. And that scene gives you a vivid illustration of the Latin roots the word comes from: ob- "against," and struere, "build." You can also obstruct something or someone by putting up a roadblock, literal or figurative: when you park yourself by the exit door, you obstruct everybody's way out.