Backfills occur when a visitor skips at least one step between the steps at which the visitor entered and exited the funnel. For example, in a funnel with four steps, when a visitor converts directly from step 2 (skipping step 3). In this case, you will see an entrance to step 2, a backfilled hit for step 3 and a hit for the funnel conversion.
To see whether hits were backfilled on the Goal Funnel Report, hover over the number of step hits shown below the step name.
You can also click on the visit log icon as the segmented visitor log shows only the visitors that actually navigated through this step.