The Greenhouse effect describes the process of external radiaton warming up the planet, because greenhouse gases are preventing the radation from being reflected back to the universe.
In detail:
Solar radiation is hitting the planet, which is a usual process. Parts of the solar radiation are being absorbed by the planet and keeping the temperature stable. The other parts are bouncing back to the universe and passing the atmosphere of our planet.
Since the increasing industrialization, the emission of carbon dioxide gases have dramatically increased. They accumulate in the atmosphere and block the radiation existing in the atmosphere, like a shield.
So the solar radiation cannot escape at a same level anymore that kept the climate stable over the past hundreds of years.
As a consequence of more solar radiation within the lowest atmosphere, the temperature is warming up. This effect is called the Greenhouse Effect!
But what is the long-term consequence of the Greenhouse Effect? Learn about the effect: Climate Change