An arithmetic sequence is a list of numbers with a definite pattern. If you take any number in the sequence then subtract it by the previous one, and the result is always the same or constant then it is an arithmetic sequence
For example:
1, 3, 5, 7, 9, ...
is an arithmetic sequence because the subtraction of two consecutive terms is the same.
9 - 7 = 2
7- 5 = 2
5 - 3 = 2
3 - 1 = 2