The Freeze tool locks affected vertices on a model so they cannot be modified while you sculpt. You can freeze vertices on both the base subdivision level and on sculpt layers. By default, the affected faces appear blue when frozen.
The areas you stroke appear blue to indicate they are frozen.
The Mask tool lets you paint opacity values on sculpted regions to non-destructively hide the sculpting on a layer. The Mask tool only works with sculpting layers, not with the base subdivision level. Each sculpting layer can have its own mask. See Mask sculpting on a layer.