What does the expression "terrestrial tetrapod" mean?

It means a four-legged creature living on land. Land in this context means only the groundcover, meaning, trees are excluded. Breaking down the words to its origins, we get the Latin word "terra" from terrestrial and the Greek words "tetra" and "pous" from tetrapod. "Terra" means earth, "tetra" means four while "pous" means foot. Side by side we get earth-four-foot. To make more sense, it can be interpreted as 'four foot on the earth'.