"Signal loss" is the overarching term for all the factors that lead to the company being less able to make good inferences about users. Not just the obvious consideration of "how do we serve an ad that is relevant to the user?" but for any data-driven decision that affects a user's experience.

The biggest recent cause of signal loss was Apple changing the rules for apps on their phone, but there are plenty of other causes.

The idea of a signal loss model is to identify ways to work around signal loss and still do a good job of making a decision with the data you have, when some of the data you were relying upon disappears suddenly.