The "long tail" describes the enormous number of specific, lower-volume search queries that collectively account for more total search demand than the handful of broad, high-volume terms everyone competes for.
The Shape of the Curve
A small number of broad "head" terms get searched constantly, while a much larger number of specific, longer phrases each get searched rarely — but there are so many of them that they add up to more traffic overall.
Why It's Called a Tail
Plotted on a graph, search volume drops off sharply after the top terms and stretches out into a long, thin tail of niche variations, which is where the name comes from.
Why It Still Holds Up
Search behavior has shifted toward longer, more conversational queries over time, which has made the long tail proportionally larger, not smaller, than it was in SEO's early years.
Next step: Use the Longtail Keywords Tool to start expanding a broad head term into the specific long-tail variations that make up your own tail.