The effect is as if you turned down the thermostat a little. We have learned that clouds can act to either warm or cool the Earth. High clouds are often thin and not very reflective. They let lots of the sun's warmth in. On balance, high clouds tend to warm the Earth.
Low clouds are often quite thick and reflect lots of sunlight back to space.
On balance, low clouds tend to cool the Earth. See FAQ for a more detailed discussion of these effects. A picture summarizes these cloud features.