The slang meaning is "I agree" or "I support that".
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.