The Barn Door Protocol was a command given by Tony Stark to F.R.I.D.A.Y. to activate lockdown procedures in the New Avengers Facility in anticipation of Hulk executing the Blip to reverse the damage done by Thanos and the Snap.
After Iron Man, Rocket Raccoon, and Bruce Banner finished building the Nano Gauntlet, Hulk was chosen to use the gauntlet to reverse the Snap due to his resistance to gamma radiation, which the Gauntlet would emit in large amounts when used. Before doing so, the Avengers prepared themselves for the unexpected, with Stark telling F.R.I.D.A.Y. to activate the Barn Door Protocol to lockdown the New Avengers Facility. Banner succeeded and brought back the 50% of all life that had disintegrated five years ago.