The amount of water available in a food can be described in terms of the water activity (aw).
Can you recall the aW of pure water?
Pure water has aW = 1.0.
The water activity of most fresh foods is 0.99. This means that they have a very high water content and can support a lot of microbial growth.
The salt draws out water from the meat so the aW is reduced. This makes the conditions too dry for harmful microorganisms to multiply in the salted meat.