It means "I agree, enthusiastically, with what someone else just said." Another informal (and ungrammatical) way to say the same thing is "Me, too!" In parliamentary procedure, a proposal is called a "motion." A person can't make a motion by himself, he needs to have at least one other person agree to it. It must be "seconded." It might go like this: A: "I move to appropriate $10,000 for heating system repairs." B: "I second the motion." Moderator: "The motion has been moved and seconded and is now open to debate. Who would like to speak to it?" In everyday life, conversation might use the same terms in a light-hearted way: A: "It's been a long day, let's adjourn to the Patriot Tavern." B: "I second that!" C: "Hear, hear!"

It means that they agree with you "i second that". Is a phrase that is often used in formal meetings when a person proposes something and someone else will second it which then allows a vote to take place. I guess this is a phrase that has originated from that use and is now used in everyday life.