Complex sentence: The "n" th triangle number is the number of dots in a triangle with "n" dots on a side; it is the sum of the "n" natural numbers from 1 to "n".
Split into simpler sentences: The "n" th triangle number is the number of dots in a triangle with "n" dots on a side. A triangle number is, equivalently, the sum of the "n" natural numbers from 1 to "n".