Telescoping refers to the phenomenon when many terms in a large sum cancel out—so the sum “telescopes.” For example:

\begin{equation*} (2 - 1) + (3 - 2) + (4 - 3) + \cdots + (100 - 99) + (101 - 100) = -1 + 101 \end{equation*}

because every third term looks like: \(2 + -2 = 0\text{,}\) and then \(3 + -3 = 0\) and so on.