Several parts combine into one character.

合 shows a lid (亼) closing snugly over a vessel (口) — the top is the meeting lid, the 口 below is the vessel's mouth. From a lid fitting a vessel exactly came "to fit, to match, to join, to gather."

Korean hanja-sound "hap": 合計 (hapgye, total / sum), 結合 (gyeolhap, combination), 合格 (hapgyeok, passing an exam), 組合 (johap, union / cooperative), 集合 (jiphap, a set / gathering). There is also a special use where 合 writes the volume unit "hop" (one-tenth of a doe), read not "hap" but "hop."

Mandarin: hé, 2nd tone. 合作 (hézuò, to cooperate), 合适 (héshì, suitable), 适合 (shìhé, to suit), 合同 (hétong, contract). It also means "to close / put together" — 合上 (héshàng, to close).

Japanese: on-readings ゴウ (gō), ガッ (ga'), カッ (ka'), kun あ.う (au) and あ.わせる (awaseru). 合う (au, to match), 試合 (shiai, a match / game), 合格 (gōkaku, passing), 合体 (gattai, merging), 都合 (tsugō, circumstances). It geminates to ガッ/カッ before some characters — 合戦 (kassen, battle), 合宿 (gasshuku, a training camp).

Memory aid: a lid (亼) snug on a vessel (口) — two parts fitting exactly into one.