A plus-one is the extra person you may bring to a party, wedding, or dinner when the invitation is for you and one other guest — but that other person is not named on the card. The host is saying, in a polite, slightly stylish way: you may come with someone.

A plus-oneplus-one is the extra person是那位額外的人 you may bring to a party, wedding, or dinner你可以帶去派對、婚禮或晚宴 when the invitation is for you and one other guest當邀請函上寫的是你和另一位客人 but that other person is not named on the card但那個人並沒有在卡片上被指名。The host is saying主人是在說 in a polite, slightly stylish way用一種客氣又有點時髦的方式：you may come with someone你可以帶一個人來。

That someone is often a partner, but it can be a friend, a sibling, anyone you choose; the point is the seat, not who sits in it. People use it as a noun ("I'm bringing a plus-one") and, more loosely, for the whole idea ("Do we get a plus-one?").

That someone那個人 is often a partner通常是伴侶 but it can be a friend, a sibling, anyone you choose但也可以是朋友、手足，任何你自己選的人；the point is the seat重點是那個位子 not who sits in it不是誰坐在上面。People use it as a noun大家把它當名詞用 ("I'm bringing a plus-one")（像「I'm bringing a plus-one」） and, more loosely, for the whole idea也比較鬆散地用來指整個這件事 ("Do we get a plus-one?")（像「Do we get a plus-one?」）。

It belongs to informal social English, especially around weddings and work events. Though actually, it is not a promise that every invitation works this way: some cards say "plus-one", some name both people, and some mean you only. If nobody offered you one, bringing an extra guest can look rude, not generous.

It belongs to informal social English它屬於非正式的社交英語 especially around weddings and work events特別常出現在婚禮和公司活動的場合。Though actually不過其實 it is not a promise that every invitation works this way不是每張邀請函都保證這樣運作：some cards say "plus-one"有些卡片就寫「plus-one」 some name both people有些會直接寫兩個人的名字 and some mean you only有些則是只邀你一個人。If nobody offered you one如果沒有人主動給你這個名額 bringing an extra guest自己多帶一位客人 can look rude, not generous可能會顯得沒禮貌，而不是大方。