在建 is a verb phrase meaning 'to be under construction' or 'to be currently being built'. It functions as a predicate or attributive modifier (before nouns) for buildings, infrastructure, or other projects that are in the process of construction. You'll commonly see it in news headlines, construction signs, and official documents (e.g., 在建工程 'project under construction'). It is not used as a simple verb like 在建造; it's a fixed expression used in formal contexts.