Foreshadowing solves a specific problem: an ending that arrives with a new element in it reads as a cheat, however satisfying the element is. Planting the element early costs a few seconds and converts a contrivance into an inevitability. It is the cheapest structural repair available.
The craft is in the weight. Planted too heavily, the audience sees the ending coming and spends the middle waiting rather than watching. Planted too lightly, nobody registers it and the payoff lands as a coincidence anyway. The usual solution is to plant it while the audience’s attention is on something else — a fact mentioned during an argument about something more urgent.
The distinction from a red herring is intent rather than form. A red herring is a plant that deliberately points at the wrong conclusion, and it only works if the real plant is also there.
What is foreshadowing?
Placing an element early so that its later importance feels earned. It converts what would read as a contrivance into something that reads as inevitable.
How obvious should foreshadowing be?
Visible but not the focus. The reliable method is to plant it while the audience’s attention is on something more urgent, so they register it without tracking it.