NFL free agency starts tomorrow, or on Wednesday -- depending on what you're looking at.
On Monday, March 7, NFL teams can talk to and negotiate with agents of players who are set to be free agents. They can't talk to these players directly, and they can't reach final agreements nor have players sign them, but they can effectively get deals done. Which is how free agents sign minutes after free agency opens officially.
Speaking of, free agency proper starts on Wednesday, March 9 at 4:00 p.m. ET. At that time, NFL teams can talk to free agents, bring them in for visits and get them to sign actual contracts. You'll see the top free agents come off the market rapidly, as most of the legwork will have already been done by then.
In reality, teams speak to free agents (or at least their agents) well before free agency opens and these dates aren't strictly observed. But those dates are when things that have already been decided, become public.