Your car is always in "negative boost" unless it's making boost.
Let's think of it this way. I'll try to kind of explain how it works and what it is. It'll be short and choppy because I'm on my phone.
Your altitude and location the absolute atmospheric pressure is 14psi. So if your boost gauge reads 0 you are filling the intake at the rate the Pistons are drawing the air out. If it's a positive number your turbo is pushing more air than the engine is pulling from the manifold.
Now, with the negative boost pressure. As the Piston moves down for the intake stroke it pulls a volume of air out of the intake. It does this by vacuum. The piston moving down creates a vacuum in the cylinder, the higher pressure air in the intake rushes to that area and a creates a loss of pressure in the manifold (negative boost) which in turn draws air through the intake. So at idle your car will read something like -11psi of boost. That's just the vacuum created by the engine drawing air out of the intake system.
The only time I've been told to be concerned about the number is if it is close to 0. It could mean the cylinder is pulling air from elsewhere. Either bad piston, rings, cylinder wall, head gasket, or failed manifold gasket.
Just the raw atmosphere is 14.7 psi. Or 1 bar. The manifold has a map sensor or manifold absolute pressure. At 0 bar it is a perfect vacuum. Your boost gauge will read -14.7 psi or -1 bar. At 29.4 psi it will show 14.7 psi of boost or 1 bar.
If the space in the cylinder, the manifold, and the air around it is all 14.7 psi air doesn't move.