Anything that's obstructive gets in the way or blocks the flow of something. If you have an obstructive object stuck in your kitchen sink drain, you might have to call a plumber to get the water flowing again.
Parking your car in an obstructive way in a parking lot will make it impossible for anyone else to get in or out. And if something's obstructive in your digestive system, it can cause serious problems. A person can act in an obstructive way too, by making processes difficult: "Your obstructive actions meant the city council couldn't even vote on whether or not to open a new zoo." The Latin root is obstruere, "block, stop, or hinder."