You can count by twos by either:

Adding 2 to the previous number or

Counting and skipping every other number.
The numbers that you would count if you started with 0 and counted by
twos would be:
0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30 and so on.
Notice that all of the counts are even numbers.
The numbers that you would count if you started with 1 and counted by
twos would be:
1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31 and so on.
Notice that all of the counts are odd numbers.