结 jié is a pictophonetic compound: the silk radical 纟 (sī) carries the meaning, and 吉 (jí) supplies the sound. The left element 纟 is the bound, twisted-thread form of 丝 sī (silk), the radical that marks dozens of characters about cord, weaving, and binding. On the right, 吉 jí (auspicious) is the phonetic, close in sound to the whole. The simplified 结 and the traditional 結 differ only in that left radical: simplified 纟 against traditional 糹, the same silk element drawn in full.

The core image is concrete: threads pulled tight into a knot. From the knot in silk, every sense of 结 unwinds. The character is polyphonic, splitting into two readings that share this root image:

jié covers the tying, joining, and concluding senses, the everyday literary readings.
jiē covers the agricultural sense of a plant tying fruit onto its branch, plus a few colloquial words.

Three streams of meaning run through the jié reading:

1. To tie, to knot. The literal act: 打结 dǎ jié (to tie a knot), 领结 lǐngjié (bow tie), 结绳 jié shéng (to knot cords).
2. To join, to unite. Two things bound into one: 结合 jiéhé (to combine), 结婚 jiéhūn (to marry), 团结 tuánjié (to unite).
3. To conclude, to settle. Tying off the loose end of a process: 结束 jiéshù (to end), 结论 jiélùn (conclusion), 结账 jiézhàng (to settle the bill).

The thread through all of them is the knot that holds: a bond made fast, an account closed, a matter tied shut. To 结 something is to bring its loose ends together and fix them in place.

The same knot that joins two threads can also tie off the end of one. In this stream, 结 means to bring a process to a clean stop: to finish, to draw a conclusion, to close an account. 结束 jiéshù ends an event, 结论 jiélùn states the conclusion of an argument, 结账 jiézhàng squares the bill, and 总结 zǒngjié sums up what came before. Each one is the loose end of something tied shut so nothing more dangles.

Watch the high-frequency word 结果, which lives in two of this character's senses at once. Read jiéguǒ it is a noun meaning "result, outcome," the thing a process ties off into: 实验的结果 "the result of the experiment." Read jiē guǒ with the second reading it is a verb meaning "to bear fruit." The settling sense and the fruiting sense meet in this one written word, which is why the reading you choose tells the listener whether you mean an outcome or an actual fruit on a branch.

The second reading, jiē, is the down-to-earth one. Its home is the orchard: when a tree ties its blossoms into fruit, it 结 jiē. 树上结了很多苹果 means "the tree has borne many apples," and here 结果 is read jiē guǒ, a verb, the plant tying fruit onto its branch. This is the original literal force of the knot image applied to growing things, the bond between branch and fruit.

From sturdiness comes the second jiē word, 结实 jiēshi, meaning solid, firm, well-built, said of a rope, a chair, or a person's robust frame: 这把椅子很结实 "this chair is very sturdy." Note the contrast in reading once more: with the jié reading, 结实 jiéshí would literally be "to set fruit," but the everyday adjective "sturdy" takes the jiē reading with a neutral-tone second syllable, jiēshi. The reading you hear tells you whether someone means an outcome, an argument, a marriage, or a firm and solid thing.

Hold the silk radical 纟 in mind and watch a thread pull into a knot. That knot is the whole character. Tie two threads and you have a bond: a union, a marriage, a people held together (结合, 结婚, 团结). Tie off the end of one and you have closure: an ending, a conclusion, a settled bill (结束, 结论, 结账). Tie fruit onto a branch and the tree bears (结果 jiē guǒ); tie a thing together firmly and it is solid (结实 jiēshi).

Let the reading guide you. The rising jié ties bonds and ties off ends, the literary work of joining and concluding. The level jiē ties fruit to a branch and ties a thing together strong, the earthier work of the orchard and the well-built. One written word, two knots: hear which one, and you know whether 结 is binding people, closing a matter, or bearing fruit.