Conversion funnel
A conversion funnel is the sequence of steps between a first visit and a completed goal, described as stages so you can see how many people leave at each one.
It is drawn as a funnel because every stage is smaller than the one before it: more people see an offer than click it, more click it than fill in a form, more fill in a form than pay. The shape is not a failure — it is the normal geometry of buying anything.
What the model is actually for is locating the narrowest transition. A funnel that loses 90% between visit and form and 10% between form and sale has a page problem, not a sales problem. Reading the stages in order is what tells you which.
A common mistake is measuring the funnel as a snapshot of who is in each stage today, rather than as a cohort — the people who entered in a given period, followed forward. The snapshot tells you what your pipeline looks like; only the cohort tells you what actually happens to a lead.