The common difference of an AP is the difference between its consecutive terms.
For the AP,
1, 2, 3, 4, ...
The common difference
= (2-1) = (3-2) = (4-3) = 1
For the AP,
5, 10, 15, 20, ...
The common difference
= (10-5) = (15-10) = (20-15) = 5
For the AP,
2, 2.5, 3, 3.5, ...
The common difference
= (2.5-2) = (3-2.5) = (3.5-3) = 0.5