Picture a shop where every box has a small paper label tied to it, and every box with the same label sits on the same shelf. A hashtag is that label, and the # is the string that ties it on. Write #NightMarket under your photo and the photo moves onto the night-market shelf, where a stranger can walk up and see it. From that one picture everything else follows: the label itself, the act of tying it on, and the piece of string on its own.
From the word’s history
From earlier hash tag: hash (the # sign) + tag (a label). The # was proposed on Twitter in 2007 as a way of grouping messages, borrowed from the channel names used in IRC chat.
早期寫成兩個字 hash tag，也就是 hash（# 符號）加 tag（標籤）。2007 年有人提議在 Twitter 上用 # 把同一話題的訊息聚成一組，做法是從網路聊天室 IRC 的頻道名稱借來的。所以「符號＋標籤」這個組合本身就是它的意思。
Wiktionary, "hashtag"
登入就可以看每一支怎麼連回核心畫面。