It can be used to express admiration or surprise at something that is impressive, unusual, or unexpected.
For example, you might say:
* "Nyumba hii ni ajabu!" (This house is wonderful!)
* "Mtazamo kutoka hapa ni ajabu sana." (The view from here is amazing.)
* "Aliimba wimbo huo kwa ajabu." (He sang that song wonderfully.)