What is a Brown Bag Seminar?

It means it's held around lunchtime and they aren't going to be providing lunch, but you are welcome to bring your own and eat during the event. So the event is somewhat informal.

On top of the other answer:

Brown bags stem from the times when people used to bring their lunch to work in a literal paper brown bag.

The events have generally transformed into (the key points being):

• a "lunch provided" or "bring your own lunch"
• are generally over one's lunch hour which is typically unpaid.

Food is often provided in order to counteract the second point of it being in your own time, as to help with attendance.

I have up-voted Elizabeth Henning's answer, but to expand a bit for those unfamiliar with the concept:

Related term: "lunch and learn"

Generally held at or around lunch time.
Will often be considered as not part of work time. Whether they are optional depends on where you work - they aren't always optional (sometimes you're expected to attend a certain number per year for continuing education).

Often it is person within your institution who does the presentation - instead of a person brought in to present to you.
The presentation isn't always directly related to your (or their) work and may be completely unrelated.

Generally where I've worked it has been considered polite to attend and act interested - but my experience with them has been more at the "less than 20 people" co-worker/department level.

I have seen the frequency decrease in the last ten years or so, YMMV

Also note comments in other answers that contradict my answer