If you put a pot of water on the stove, and turn on the gas, it'll warm up.
If you turn the gas off, it'll cool down
If you put a lid on the pan, when warming, it will warm faster because it's losing less heat.
And when cooling, it will cool more slowly because it doesn't lose as much heat.
If you put a pan on the stove and turn the heat on to low setting, it will warm, but not boil.
If you put a lid on that pan, it will warm a bit more, because the lid retards heat loss.
The earth receives energy from the sun, and radiates heat away. When the amount of heat received = the amount of heat lost, you have equilibrium. If you prevent the earth from radiating as much heat away, then the earth will warm, until it loses as much heat as it gains. CO2 prevents the earth from radiating as much heat away, so it's warming, until it reaches a new equilibrium.