How far is that tree? Hold your thumb up at arm's length. Close one eye, then the other. The thumb appears to jump sideways. The bigger the jump, the closer the object.
Rule of thumb (literally)
The distance to an object ≈ 10 × the size of its sideways jump (in the same units). It's not exact, but it's a fast field estimate.
Surveyors and snipers use a more precise version called parallax. Astronomers measure distances to stars the same way.