Counting by \(2\)s means going up by 2 each term instead of by \(1\).

Here is usual counting: \[1,2,3,4,5,\dots\]

Here is counting by \(2\)s: \[2,4,6,8,10,\dots\]

In regular counting by \(1\)s you can start from any number. Here is counting by \(1\)s starting from \(7.\) \[7,8,9,10,11,12,\dots\]

Here is counting by \(2\)s starting from \(7.\) Each term goes up by \(2.\) \[7,9,11,13,15,17,\dots\]