An example of an arithmetic sequence is:
20, 22, 24, 26, ...
To find the common difference, subtract each term by the previous term.
22 - 20 = 2
24 - 22 = 2
26 - 24 = 2
Thus, the common difference is 2. It is used to find the next terms of the sequence.
26 + 2 = 28
28 + 2 = 30
30 + 2 = 32
and so on...
*If you have questions regarding this, comment below and I will answer them. ^^