Answer
It means that this infinite series converges and its sum is equal to $5$. In other words, the sum of the terms is 5 or for precision (since we cannot sum infinite number of terms and get a finite result precisely) the limit of the nth partial sum is equal to $5$. (Check page 748, definition 2)
Work Step by Step
It means that this infinite series converges and its sum is equal to $5$. In other words, the sum of the terms is 5 or for precision (since we cannot sum infinite number of terms and get a finite result precisely) the limit of the nth partial sum is equal to $5$. (Check page 748, definition 2).
FOR MORE CLARIFICATION:
Definition 2 basically explains that if the limit of the nth partial sum of a given infinite series exists and equals a real number that means that the given series is convergent and the result of that limit of the partial sum is the sum of the infinite series.
