Rearranging finitely many terms of an infinite series does not change the sum of the series because there will be some partial sum \(s_n\) that contains all of the rearranged terms, \(s_n\) is a finite sum unaffected by rearrangement, all the partial sums \(s_k\) for \(k > n\) are thus unaffected by the rearrangement, and thus \(\displaystyle\sum a_n = \lim_{n\to\infty} s_n\) is unchanged by rearranging finitely many terms.
Rearranging infinitely many terms of a conditionally convergent series can change the sum of the series! When you rearrange infinitely many terms, you can change the value of infinitely many partial sums (and hence the sum of the series).
Rearranging infinitely many terms of an absolutely convergent series does not change the sum of the series!