Lukewarm is that temperature, often referring to water, which is between cold and warm. You wouldn’t say it’s cold, but then you wouldn’t say it’s warm either. The word is not used to refer to the temperature outdoors, however.
Here are a few more examples:
« Add lukewarm water to the dry yeast, and wait for it to bubble. »
« Is the water hot? No, it’s only lukewarm. »
« The soup is lukewarm, so the cheese won’t melt. »