Being warm-blooded was having the ability to maintain a constant body temperature. Mammals and birds were examples of warm-blooded creatures, also known as endotherms. The opposite of warm-blooded creatures were cold-blooded creatures.
The last common ancestor to warm- and cold-blooded organisms on Earth
The Eryops, a creature that lived over four hundred million years ago was, according to The Doctor, "believed to be the last common ancestor of cold-blooded and warm-blooded organisms" on Earth. (VOY: "Distant Origin")
In Doctor Gillian Taylor's presentation, "The Wonderful World of Whales", she explained that "the first commonly held misconception is that whales are fish. They're not, they're mammals just like you and me. Warm-blooded, needing air to breathe and producing milk to nurse their young." (Star Trek IV: The Voyage Home)
Examples of living creatures that were warm-blooded included Humans. (TOS: "Arena") While analyzing the bones of Hogan, which were discovered on Hanon IV while investigating the Distant Origin Theory, the Voth scientist, Forra Gegen, noted that "his cartilaginous microstructure is extremely porous," indicating that he was "most likely warm-blooded." (VOY: "Distant Origin")