KanjiJLPT N1
Outline

outline, condition, approximation, generally

Mnemonic

A tree that has already shed every leaf reveals its true outline against the winter sky. Without foliage obscuring it, you see only the broad, sweeping shape — the gist of the tree, stripped of all detail. That bare silhouette is the outline of what it once was.
Additional thoughts
Picture a lone, leafless tree on a hilltop at dusk — its stark skeleton is the perfect visual for 'outline': general shape, no fine details. Already bare = already simplified down to essentials.
Quick recall
A tree that has already lost its leaves shows only its bare outline against the sky.

Details

The keyword for 概 is outline. This kanji conveys the idea of a general summary or broad overview of something, capturing the essential points without going into fine detail. It carries the sense of approximation and generality, as when one speaks of the gist or overall picture of a matter. By extension, it can also suggest a sweeping, all-encompassing perspective that smooths over particulars in favor of the bigger picture.
On'yomi
がい
Kun'yomi
おおむ.ね