PostEaseThe short answer
A content series is a recurring, recognizable format published on a regular basis: the same intro, structure, or theme each time, with new content inside it. It differs from a content pillar, which is a broad topic area, in being a repeatable format an audience learns to recognize on sight.
Why a series builds habit better than one-off posts
A recognizable, recurring format gives an audience a concrete reason to expect and look for the next installment, in a way a series of unrelated one-off posts does not. The format itself becomes the habit-forming device, independent of any single post's topic.
What makes a series sustainable
A series with a format simple enough to produce regularly, without disproportionate effort each time, tends to survive longer than an ambitious, heavily produced one that becomes a burden. Sustainability matters as much as initial appeal.