A wireframe decides only what goes where. Gray boxes stand in for buttons, lines stand in for text, a diagonal X stands in for an image — every visual decision is deliberately deferred so the conversation stays on structure and priority.
It's easy to confuse with a mockup (a finished-looking still) or a prototype (a clickable flow), but the three answer different questions: a wireframe answers "what's where," a mockup answers "how does it look," a prototype answers "how does it behave." Skip straight to high-fidelity screens and reviewers get stuck arguing about color and font instead of giving feedback on the actual structure.
Fidelity itself is a choice, from a hand-drawn sketch on paper to a pixel-aligned grayscale wireframe. Reach for the sketch when you need fast agreement on structure with stakeholders; reach for the aligned version when it has to serve as the layout spec developers build from.
When to use
Reach for it first, before a pixel is styled, whenever placement and priority aren't agreed yet. If reviewers keep commenting on color instead of layout, that's a sign to fall back to this stage.