Microbes need liquid water they can actually use. Salt and sugar take that water away by osmosis. Add some and watch the water leave the cell, the water activity fall, and one group after another drop out of the running. This is why salted meat and sugared jam keep.
One cell in the solution drawn from the number
Water always moves toward the saltier side. The cell wall keeps its shape, so what you see move is the membrane inside it.
Who can still grow lowest water activity each one tolerates
Each group has a floor. Below its floor it simply stops. Nothing at all grows below a water activity of 0.60.