On Earth we have several greenhouse gases that exist in our atmosphere: water vapor, carbon dioxide, methane, and a few more. These gases are termed "greenhouse" because they're capable of trapping energy in the form of infrared radiation, aka heat.

The sun emits energy that enters our atmosphere and reaches down to the Earth's surface. Much of that energy is reflected off the surface of Earth and re-emitted back into the atmosphere in the form of infrared radiation. Without a lot of greenhouse gases in the atmosphere, that energy that bounces off the Earth's surface would escape into space, but greenhouse gases trap that energy which in turn heats up the atmosphere.

This is generally a good thing. Without greenhouse gases, we would have a very cold atmosphere that wouldn't be conducive to supporting complex life. So we need some of these gases in our atmosphere to survive. But, since the industrial revolution in the 1800s, more and more CO₂ has been put into the atmosphere which is throwing off the balance of the right amount of greenhouse gases keeping our planet JUST warm enough without being too hot. Trapping more heat in the atmosphere also means that the oceans are getting warmer. When both of these reservoirs heat up, it can disrupt weather patterns which is why one of the "symptoms" of climate change is an increase in the number of intense weather events like longer droughts, heavier rain storms, stronger hurricanes, etc.