what do Greenhouse gases do?

Basically what greenhouse gases do is help trap heat in the earth's atmosphere. So the more greenhouse gases there are the more heat gets trapped. The heat is trapped through a process called the greenhouse effect. The result of the greenhouse effect is global warming which causes climate change.

Climate changes are sometimes caused by natural factors but the climate changes that we are seeing and experiencing today are almost entirely caused by one important factor:

Human activities that change the atmosphere's composition (e.g. through burning fossil fuels, industrial production, etc...) These human activities increase the amount of greenhouse gases in the atmosphere which keeps more heat in our atmosphere, facilitating global warming.

The way global warming will affect our weather/climate will be different from region to region. Places near the equator won't see large changes in temperature, but one thing that is sure is that places like the Arctic Ocean and Antartica will become a lot warmer than normal. This will end up melting enormous quantities of ice and cause world ocean levels to rise, creating flooding in many costal areas (about 60% of the world's population live not far from a shoreline).