Prevention effects are very similar to replacement effects, but are specifically related to preventing damage, and apply after replacement effects.
For example, is a prevention effect. When applying damage to a Tufted Turtle, first apply any replacement effects (e.g. Lance, ) and then apply the Turtle's prevention effect last. This rule ensures that prevention effects function properly. E.g., you can't somehow apply a Lance after the Turtle's prevention effect was applied, somehow dealing damage, which would be wrong.
For more details, see the "Damage" codex entry and subsections.