Counting by $5$s means going up by 5 each term instead of by $1$.

Here is usual counting: $$1,2,3,4,\dots$$

Here is counting by $5$s: $$5,10,15,20,\dots$$

In regular counting by $1$s you can start from any number. Here is counting by $1$s starting from $8.$ $$8,9,10,11,12,13,\dots$$

Here is counting by $5$s starting from $8.$ Each term goes up by $5.$ $$8,13,18,23,28,33,\dots$$ Notice that every two-terms ends in the same number. In this example, the first, third, and fifth terms are numbers ending in $8.$ The second, fourth, and sixth terms are numbers ending in $3.$ This pattern always happens when counting by $5$s.