It is a very important measure of changes going on in Earth's machinery. A rise of just one degree Fahrenheit (°F) on a sunny day where you live has little effect. But over the whole Earth, a rise of 1°F makes a big difference. Just think, normally, water at 32°F is solid ice. But water at 33°F is liquid water.
Even a small rise in Earth's global temperature means melting ice at the North and South Poles. It means rising seas. It means flooding in some places and drought in others. It means that some plants and animals thrive while others starve. It can mean big changes for humans too.