Rafts?

[Mooselake]

Rafts were used in the early days before they understood (or maybe had) flat print surfaces (we call them beds) that were the same distance from the extruder tip across the whole surface.  Rafts filled in the dips and got around the problem.  Then they learned to use something flat (window glass works great, I print on double strength regular window glass with a thin layer of school glue stick, and have for years.

Rafts really aren't necessary any more, although like most stuff from the olden days (in 3D printing that's like 5 years ago ) they still have their fans.  There's probably rare exceptions but they're a waste of time and plastic.