You've got a good start - don't try to pretend you're something you're not. Most folks will respond much better to "I am not quite sure of the process involved here - can you go into a little more depth?" than to someone obviously trying to fake it.
If you're looking for the underpinnings of graphic design, look at things like:
- Color theory (which colors work together to create certain moods, feelings, etc.)
- Page layout (placing elements in certain ways to ensure people look at them as well as knowing how different cultures look at pages in different ways)
- Basic concepts of UI design such as redundancy (an elevator button that says "Up" as well as showing an arrow pointing upwards) and common visual cues (putting a push panel on a door that you want people to push and putting a pull bar on the other side)
- The design process, which involves generation of a concept (or several concepts), feedback (from clients, users, other designers - this is otherwise known as "watching your beloved little puppy that you've groomed and fed and raised from birth being dropped into the dog fighting ring")
- Trends in design (shudder). What's cool now? What are the major movements, schools of thought, prevailing aesthetics, etc.
Ask. Ask designers you know, look on web sites. It's okay if you don't understand or like everything you see (I sure as heck don't). Challenge things that don't seem right. Get opinions from diverse groups of people.