Asam keping are the sun-dried slices of fruit from the Asam gelugor Garcinia atroviridus tree, native to Malaysia. The very sour fruit is commonly used in Malaysian cooking. The rind of the asam gelugor is particularly sour. The rind is cut in thin slices and dried and added to recipes that require the sour flavor profile.
The tree has medicinal uses as well. A decoction of the asam gelugor is used to treat cholesterol and high blood pressure.
The tree is related to the mangosteen (garcinia mangostana) and the fruit look very similar but the asam gelugor fruit is bright yellow rather than purple like the mangosteen.
The image shown is a dried cross section slice of the fruit. For more information about this fruit see the Globinmed site.
If you can't find the asam gelugor (rind) you can substitute: