What does the seeds on challah bread symbolize?

An interesting question. I looked it up; and the consensus is that the braiding of the Challah represents unity, intertwining, the central point, innerness, connectedness.

The three braids could hint to any of the "threes" in Judaism: kohen Levi Yisroel (the three divisions of families), Torah Neviim Ketuvim (Jewish Bible), the three Sabbath meals, etc.