Superbly simple question but requires a more complex answer.
Infrared (IR) is light. Light comes in many colours as we can see in rainbows which break white sunlight into all the different colours. Infrared is a colour that is dark, dark red... so dark that our eyes cannot see it but it is still there. In fact we can 'feel' infrared when we stand near a hot radiator and we feel warmth 'radiating' from the 'radiator'.
All types of light is energy radiating away from objects and things around us and the colour and brightness of this light is mostly linked to how hot the object is. A simple experiment with a lightbulb shows us that when it is slowly lit up with a dimmer control, it starts to glow red, then orange, then yellow, then white while also getting brighter as you go ... if we could put more electricity into it to make it even hotter without meltiing the metal filament, then the bulb would glow blue with great energy and brightness... just like the hottest blue stars in the night sky.
Upon switching off the lightbulb, it is still hot to touch... this is the infrared that we can 'feel' but we cannot see.
The cooler a star glows the dimmer it will look and the more white-orange-red it appears for a given star size.
Our eyes only see a small bit of all the different colours of light that exists in the universe but we do see a lot of the colours of sunlight that travels from our sun through space and our thick atmosphere which acts like a big light filter, which is really handy and thanks to evolution of eyes over millions of years.
Infra red is just too dark red to see with our eyes, on the other side of the rainbow is ultraviolet that is so dark dark blue we just cant see it either. Snakes can feel infrared heat with their noses from warm animal bodies on a cold night to help them hunt and bees can see ultraviolet light reflecting of flower petals to help them find nectar to make tasty honey.