To second someone is to publicly express support for a proposal he has made. Since he is the first to raise the idea, you are the second to support it - hence you "second" him or his idea. E.g.: Geraldine (to a group): Why don't we spend the funds on marketing? Thomas: I second Geraldine. / I second Geraldine's suggestion. / I second that.

Second mainly means to support or back someone up. But It has another meaning just close to the first meaning, it means to agree with someone.