The use of infrared devices for fighting during nighttime began during the last part of WW2. Both the Germans and the Western Allies (UK and USA) had infrared equipment. It was active - meaning you had to have a searchlight emitting IR light and then tanks, weapons etc. had devices that could "see" the IR light.
The Germans began producing camouflage uniforms during the very last part of WW2 that had special colours to make the pattern more efficient viewed under IR light (eg. the Leibermuster. Other patterns - eg. Zeltbahns - were overprinted with carbon black.
What is IR light? IR light has a wavelength that is higher than the visual light that the human eye can see. There is plenty of IR light emitted by the sun. For more on IR light: Article on infrared on Wikipedia
Most patterns simply look like a black & white image of the visual pattern - but there are exceptions. When viewed under IR light this Czechoslovak pattern looks completely different:
You might wonder why they the Czechoslovak Army has chosen to have a special pattern displayed only under IR light when they have chosen the "rain pattern" for visual light... BTW, when viewing a Czechoslovak uniform in this pattern you can faintly see the IR pattern, especially on the reverse side of the clothing.