Lockdown is a book about a boy in prison—it has handcuffs on the cover, so it's not exactly a secret what the title is about. But while "lockdown" is a broad reference to incarceration, it also has a special meaning within the facility: It's when inmates have to sit quietly in their cells (sort of the prison equivalent of being sent to your room).
Reese doesn't mind that quiet time. It keeps him from getting into fistfights, for one thing, and while he doesn't like being in Progress (at all), he recognizes that it offers him sanctuary from the real world, where there are too many opportunities for him to get into trouble:
When I first got to Progress, it freaked me out to be locked in a room and unable to get out. But after a while, when you got to thinking about it, you knew nobody could get in, either. That was the cool part about being in Progress. You were in lockdown but you were also shutting the world out. (3.27)
Lockdown teaches Reese to look inward instead of focusing on his external surroundings. And, ironically enough, this is exactly what he'll need to do as a free man after his release. He needs to rely on himself instead of getting distracted by the bad things going on around him.