A great question! Lots of things happen really, but I will talk about two effects that are pretty different.

The first is heating things up and the second is the discoloration of things left out in the sun for too long (e.g., if you've ever left out drawings or crafts like I have).

We will see that both originate from two different effects of the same thing:
First, a little physics.

Light, as you will learn in physics class, is a form of energy. It is useful in this context to think of light as wave, with some frequency (how wiggly it is) and amplitude (how big the wiggles are).

We learned in the early 1900s that as the frequency of light increases, so does the energy it carries. It turns out, light can exist on many, many frequencies that span what is known as the electromagnetic spectrum.

The inverse of the frequency is the wavelength, so small frequency equals large wavelength. Only a small part of the electromagnetic spectrum is visible to the human eye. We recognize this as color. However, there are many, many wavelengths (aka frequencies) of light that range from the length of an athletic field to that of an atom.

Light from the sun spans a pretty large chunk of the electromagnetic spectrum. You'll notice a small part is called UV (this is the part of the spectrum sun screen protects against). A sizeable chunk is in the visible (that what let's us see things in the day), and a huge portion is in the infrared. Infrared is essentially energy in the form of heat, which is why things get warmer under the sun.

Things discolor mainly because of the UV. UV light is high in energy, so it is able to cause chemical reactions that leads to changes in color.

There are of course a whole host of things that can happen when sunlight hits an object that is entirely dependent on the properties of that material.

In fact, optimizing the amount of light that is absorbed is a key materials science challenge for solar cells! Nevertheless, it all comes down to the electromagnetic spectrum of the sun.

Hope this helps!
Best,

Sunlight includes ultraviolet light and powerful visible light that tend to break down chemical bonds. This causes dyes to bleach, and eventually destroys material left out in the sun.

When objects are exposed to sunlight, three things can happen:

reflection, transmission, and absorption.

Reflection is when light bounces off a surface. This is what allows you to look at yourself with a mirror.

Transmission is when light goes through a surface. This is what allows you to look through windows.

Absorption is when light energy is used to do something useful or harmful to the object. For example, plants absorb this energy and use it to make food. Lizards and snakes absorb light to heat up their bodies. Humans need sunlight to make Vitamin D. If sunlight hits a solar panel, it can evengenerate electricity. Unfortunately, absorbed sunlight can be harmful too.

The absorbed energy can cause unwanted chemical reactions. For example, milk will go bad faster if exposed to sunlight. Too much sunlight can also be harmful to humans. This is why we wear sunblock. We need to protect our skin from the harmful UV light.