The yellow arrow in the above diagram shows the heat radiation from the Sun coming down toward a greenhouse on the surface of the Earth.
Because the Sun is so hot, the radiation can easily get through the glass panes of the greenhouse.
The inside of the greenhouse absorbs the Sun's radiation and starts to heat up, producing radiation of its own.
However, because the inside of the greenhouse is much cooler than the surface temperature of the Sun, it produces a different sort of light (infra-red) that does not pass through glass so easily. Thus the heat gets trapped inside.