Why are carbon (IV) oxide and methane called greenhouse gases?

Carbon (IV) oxide, or carbon dioxide, and methane are called greenhouse gases because they contribute to the greenhouse effect.
The greenhouse effect is caused due to higher transparency of our atmosphere to the solar radiations in the visible range, which allows the radiations to pass through it. However, the infrared radiations emitted by the warm surface of the Earth are trapped by the atmosphere. This causes a warming effect.
The greenhouse effect is the reason that our planet has a more comfortable average temperature of 15 degrees Celsius. In absence of the greenhouse effect, the same would have been about -18 degrees Celsius.
The greenhouse effect is caused due to the presence of certain gases in the Earth's atmosphere, including carbon dioxide, methane, nitrous oxide, ozone, and others.
The reason we are worried about greenhouse gases is due to the increase in their atmospheric levels owing to anthropogenic activities such as the burning of fossil fuels, deforestation, changes in land use, and so on. This increase has resulted in an increase in the average temperature of Earth's surface.