Liquid or fluid web pages expand and contract with the width of a visiting browser.
They also conform to the various different monitor resolutions used by the different devices that are used to view them.
Grab the right frame of the browser and push it to the left. Watch as the web page conforms to the width of the browser window.
Then test the page at different monitor resolutions.
If the page was set to a static or fixed width of 1000 pixels, it would require a visitor viewing on a tablet set to 800x600 to scroll unmercifully to the right to view your page.
A person viewing on a high end game machine at 1920x1200 would only see about one half of the screen on their monitor occupied by your web page.