Core sense is tying threads into a knot - extended to forming bonds (结婚, 结盟), congealing (结冰, 结晶), and wrapping things up (结束, 结账, 结论). The jiē reading is reserved for plants bearing fruit - see the other entry.
memory
Picture a silk thread (纟) being pulled into an auspicious knot (吉) - every 结 sense (marriage, alliance, conclusion, decorative knot) follows from that single image.
Silk radical on the left - the left-side form of 糸. Direct semantic anchor: 结 is literally to tie a knot in thread, from which all modern senses radiate (knot, bond, settle, conclude, freeze over). Same family as 纸, 织, 线.
Right side 吉 supplies the sound - jí shifts to jié with a regular coda. Same phonetic in 洁 (clean) and 桔 - the j- onset stays put. A faint semantic flavour fits too: tying the marriage knot 结婚 carries an auspicious overtone.
