Picture two working tools laid across each other: a hammer, the tool of someone in a factory, and a sickle, the curved blade of someone in a field. Put one over the other on a red ground and you have a single mark that says these two kinds of workers belong together. Every use of the phrase starts from that crossed pair — the mark itself, then the flag it was printed on, then the system that flew the flag.
From the word’s history
The supplied source says the design was made by Yevgeny Kamzolkin in 1917, built on earlier pairs such as the hammer and pick of mining, with the hammer for factory workers and the sickle for farm workers. That reading is exactly what the crossed image still shows.
本批資料所附的說明指出，這個圖案由 Yevgeny Kamzolkin 在 1917 年設計，參考了更早的「錘子與鎬」（採礦象徵）；錘子代表工人，鐮刀代表農人。今天看這個交叉的圖案，看到的仍然是同一件事。
Wiktionary, "hammer and sickle"
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