In a nutshell...
The annual MLB "Winter Meetings" are held each December where representatives from the 30 major league baseball teams and their minor league affiliates meet to discuss business and hopefully conduct off-season trades and transactions.
Usually attending are team owners, executives, GM's, and scouts.
There is a trade show where exhibitors can show their wares as well.
During the last day of the meetings, minor league players can be drafted by major league teams.
In 2014, the meetings were held in San Diego, this year, December 7-10 in Nashville.