The DDM term is mostly used in the GNSS Reflectometry (GNSS-R) scope, but is conceptually equivalent to the radar ambiguity function. In fact, a GNSS-R system can be understood as a bistatic radar, that is to say, a configuration in which the transmitter (the GPS/Galileo/GLONASS/Beidou satellite) and the receiver (which receives the reflected signal) are different vehicles.
The bistatic scattering has its own complexities, but a GNSS-R instrument can operate as a radar altimeter, scatterometer, or even as a SAR radar . People get often confused with the GNSS-R concept, but essentially the idea behind GNSS-R is to use the GNSS reflected signals as "a signals of opportunity". It is unrelated with the original navigation/location purposes of GNSS. In fact, in GNSS-R, the navigation messages are not demodulated, except maybe, in order to remove phase jumps due to data bit transitions.
The term DDM comes from the fact that reflections are mapped into a delay-Doppler plane. Reflections over different areas but a the same distance to the receiver and with the same Doppler shift, are mapped into the same "pixel" in the DDM. This is why in radar this function is called ambiguity function, because in principle, it is not possible to locate the reflection in the spatial domain, except for the specular reflection. Although there are methods to resolve or to reduce this ambiguity.
Picture from Thesis PhD, Alejandro Egido, GNSS Reflectometry for Land Remote Sensing Applications, July 2013, DOI: 10.13140/RG.2.1.2078.7049
 V. U. Zavorotny, S. Gleason, E. Cardellach, and A. Camps, “Tutorial on Remote Sensing Using GNSS Bistatic Radar of Opportunity,” Geoscience and Remote Sensing Magazine, IEEE, vol. 2, no. 4, pp. 8–45, dec 2014 https://ieeexplore.ieee.org/document/6985926