There's a laser that travels down the rods, and all the air is sucked out the tunnels to make a vacuum. The setup is very sensitive, gravitational waves will cause the laser to stretch or shrink and is what is measured by the computers. There's an observatory in both Washington and Louisiana. The two facilities compare their data to make sure that they are actually measuring gravitational waves instead of nearby earthquakes and cars driving by etc.
The Laser Interferometer requires vacuum for an infrared light to bounce off a mirror to accurately measure fluctuation of 1/1000 the width of a proton in the distance of each "rod". In short, it is a really expensive tape measure, that requires isolation from unwanted signals. Theory predicts changes in spacetime would be seen as gravitational waves when certain events occur in the universe such as the merger of two black holes. The two rods provide data in different directions. This measurement and data collected elsewhere in the world can be correlated and then compared with a numerical model of general relativity. The two rods are the concrete housing for multiple layers of engineering to suppress planetary interference. The inner chamber is the second largest vacuum on the planet after the Large Hadron Collider. The rods are the culmination of fifty years research to provide experimental data predicted by Einstein one hundred years ago.
My attempt at layman's explanation: It uses light to measure distance. When a gravitational wave passes, it should stretch and shrink the space along one tube more than the other. That makes the light combined from the tubes jitter. The sister site and the European one make local effects ignorable, so we know jitter results really are from far far away, and maybe even from which direction they come.
I recommend when you get an opportunity to visit the site and to enjoy all the neat things to visit in the vicinity I am not a scientist but when I took the tour I had a greater understanding of what was happening it was truly a neat experience
https://youtu.be/sCuKuUgNfjA Check out this video from Linus Tech Tips that goes into detail about what goes in to the measurement of gravitational waves. It's pretty interesting.
Well! Your suggestions are really helpful but I am living in India so it is not possible for me to visit the site
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