The consumer price index is a weighted basket of things households buy. The Bureau of Labor Statistics prices the basket and publishes an index. The number people quote as 'inflation' is usually a percent change in that index over a month or a year.
The distinction matters when you compare months. A high index level can coexist with a small monthly change, and a low-looking level can still be rising quickly. Read the period the release names. 'September' on a CPI story is the month being measured, not necessarily the morning the number was published.