Crunching Knowledge is the act of investigating knowledge, with the goal of extracting the relevant information, and coming up with good fitting Domain Models. Sifting through the noise to find the signals.
In Domain Driven Design this technique refers especially to using the knowledge of domain experts, by walking with them through scenarios, to refine Domain Models, their relation, interaction, etc
Crunching Knowledge is an iterative, continuous process. While it is one of the first and most important things, it must continue throughout the whole process.
The initial source from which the Ubiquitous Language emerges.