When you condense something, you cut it down and trim it. You may love every word of your 1000-page novel, but you'll have to condense the plot into a 2-page summary for your editor.
When you condense something, you make it smaller and more compact. If you condense your wardrobe, you get rid things and keep only what you wear most. When a gas or vapor turns to liquid, it condenses. When you take a long, hot shower, the steam condenses in your bathroom, your mirror clouds up, and everything gets damp. Condense comes from Latin con-, “completely” and densus, “thick, dense."