Complex sentence: The usual choice is for "I" to be infinite and "U" to contain all cofinite subsets of "I", otherwise the ultrafilter is principal, and the ultraproduct is isomorphic to one of the factors.
Split into simpler sentences: The usual choice is for "I" to be infinite and "U" to contain all cofinite subsets of "I". Otherwise the ultrafilter is principal, and the ultraproduct is isomorphic to one of the factors.