The real meaning comes from parliamentary procedure, the way in which a group of people come together and most efficiently present and discuss possible courses of action, and make decisions. "Roberts Rules of Order" has been the standard of our government, judicial system, and formal organizations since 1876.
One member may stand and address the chairman. Once recognized, the member makes the motion: "I move that/to..." and resumes his seat. Some types of motions require another member, without rising, to second the motion: "I second the motion," or "I second it" or even just "second." Once seconded, the motion becomes a topic of organized conversation until two-thirds agree to vote. Seconds are important because some topics are not worth the group's time to discuss and a 2nd person means the topic is important to more than one person. Technically, the 2nd does not have to support the motion, they are simply agreeing that it should be a topic of discussion.
Tune in to any congressional session on CSPAN or attend your city council meetings to see Roberts Rules of Order in action.
The Temptations song "I second that emotion" is a pun on Roberts Rules of Order.
"I second that."
In this slang use, it means support of the statement but it stays just a complaint. However, if the board were using real parliamentary procedures, the seconded motion would invite others to post additional comments and would remain a live topic until a vote is called.
"I move that images on the front page be limited to 200kb."
"I second that motion."
"Dial-up users are burdened with file sizes that large when twenty or thirty of them must be loaded at once."
"People can put links to bigger images instead of the whole thing messing up the way the text flows."
"Only 5% of our members are dial-up; we should be able to post whatever we want for the majority of the users who are high-speed."
(Vote: aye/yay/yes/hands or no/nay/hands)
"Yays have it, motion is carried that images on the main page be limited to 200kb or less." (This is now a rule.)