In short, functional requirements are functional features that users expect from a proposed software product. Non-functional requirements, on the other hand, are more related to security, performance, and the look and feel of the user interface, a.k.a. (UI). For a project to be successful, it is critically essential that both the customer and the Google project delivery teams have a clear definition and agreement on what the project scope and detailed requirements are going to be.

There are three types of requirements: conscious, unconscious, and unidentified or TBD requirements.
When the Google hiring manager asks this question, they want to know if part of your job requires gathering and analyzing functional or non-functional requirements. For functional requirements gathering, they will want to hear how the system behaves under certain conditions within its environment, how it responds to inputs, and what outputs it generates. As far as non-functional requirements, they will want to also hear things like constraints on the system, quality attributes, security, performance, and availability.