The "greenhouse effect" keeps Earth warm enough for us to live and grow food. But too much would be bad.
Some gases ("greenhouse gases") let sunlight in, which warms the Earth, and then block that heat from leaving. That's the "greenhouse effect", and it's a natural thing, mostly caused by water vapor.
Man is making excessive amounts of greenhouse gases, mostly by burning fossil fuels. That causes the delicate natural balance to go out of whack and the Earth warms. That's global warming.
It won't be a Hollywood style disaster. Gradually coastal areas will flood and agriculture will be damaged. But it will be very bad. Rich countries will cope, but it will take huge amounts of money. In poor countries many people will die of starvation, but not all of them.
Most scientists say, in 20-50 years. But we need to start right now to fix it, fixing it will take even longer than that. thank you