When: The legal tampering period starts tonight at midnight (EST) and ends at 4:00 p.m. on Tuesday.
Who: Teams can contact the agents for players but not the players themselves. Players without agents can not negotiate with teams. This applies to unrestricted free agents only. Not restricted free agents or exclusive rights free agents. See the full list of Chiefs free agents here.
What: Teams can not execute contracts with player agents. They can negotiate and set the parameters for a deal, but they can not actually execute the contracts until free agency starts at 3:00 p.m. on Tuesday.
Where: Nowhere. As in, players can not meet with teams until Tuesday. Agents can talk but visits can't happen until Tuesday.
The NFL posted a Q&A for those that have more questions on the subject.
Ed Reed won't be able to negotiate at all during this time. He is one of the few players that elects to broker his own deals without the aid of an agent. After the three-day period ends, teams will be able to schedule visits from prospective free agent acquisitions.