To 「second」 something means to be in support of a proposed idea or motion.
second某事的意思是支持一個提議或想法。
Formally, in a meeting when someone suggests or proposes an idea, there needs to be a 「second」 (someone else who supports the motion) for discussion to take place that results in a vote. Without a 「second」 the idea or motion dies and is not considered.
正式地說，在會議上，當有人提出一個建議或想法時，需要有第二個人(支持該動議的其他人)進行討論，然後進行投票。沒有second的想法或動議則不被考慮。
Informally, if people are just talking in a group and someone has an idea or makes a point about any particular subject, someone else in the group may say, 「I second that」, meaning that he/she supports the idea and agrees with what was said.
非正式地說，如果人們只是在一群人中交談，有人對任何特定的主題有一個想法或提出一個觀點，這個群體中的其他人可能會說，I second that，意思是他支持這個觀點並同意所講的內容。
例：
A：How about pizza for dinner?
今晚吃披薩怎麼樣?
B：I second that!
我同意。
The motion was proposed by the club's chairwoman and seconded by the secretary.
該動議由俱樂部女主席提出，由幹事附議。
"I could use a drink." "I'll second that (= I agree with you)!"
「我想喝杯飲料。」「我贊成！」