I didn't know what it was either, but I googled it and found a simple explanation on Wikipedia. I'm sure the interviewer would at least give you that basic background information ("Pascal's triangle is a structure in which each number in the triangle is the sum of the two directly above it.")

I don't think the point is that you would be expected to know every historical abstract concept, but rather that given the basics of one, you could write code to implement a data structure around it.