A greenhouse gas is an atmospheric gas that can absorb and then emit infrared radiation. The main ones include water vapor, carbon dioxide. nitrous oxide, and methane. The concentration of these gases in the atmosphere affect global energy levels. Increasing concentrations lead to higher energy levels.
The ones we are particularly concerned about today are carbon dioxide, methane, and nitrous oxide, because concentrations of all of these are increasing due to human activities such as agriculture, burning of fossil fuels, and land use changes (especially forest clearing).
While natural systems have some capacity to absorb and mitigate environmental changes, they can only do so much, especially when those changes happen rapidly. We are currently altering the global energy balance at an unprecedented rate.
There isn't total agreement about what the ultimate consequences of these changes will be, but most climate scientists, in my opinion, believe immediate and urgent action is needed to halt the increasing concentrations of these gases.
Personally, I think, that at the rate we are going, we will be lucky to last a century. We can't even sit down and agree what color the sky is.