I think in a formal meeting if some one proposes something there needs to be a voting member to be a "seconder", this shows for the record (minutes) that there is support for the proposal. Obviously if there were no seconder, there is no support for the proposal, no voting is necessary. The regulations of the meeting might require that a vote be cast and some majority of supporters must be counted for the proposal to go ahead. So there can be more than one supporter, but only one is required to be the seconder.
The above idea may be carried over for mundane use, jokingly. For example someone in the office suggests that they finish work for the day and meet at a local pub for a quick drink. Another worker says "I'll second that".... meaning that she supports the idea.