Significance and properties of lossy silicon substrate

It means it has a high conductivity (e.g. 50Ohmcm) compared to GaN, for example. It influences among other the quality factor of passive components.

Silicon substrate is conductive, and current will flow in the substrate.
This is different from isolating MMIC substrates (GaAs etc).

One area where you see the effect of the conducting silicon substrate is inductors: at high frequencies, the inductor performance (Q factor) is limited by coupling to the conducting substrate.