What exactly shared means?

[John Colvin via Digitalmars-d-learn]

On Friday, 2 January 2015 at 22:10:36 UTC, Ola Fosheim Grøstad 
wrote:
> On Friday, 2 January 2015 at 21:06:03 UTC, John Colvin wrote:
>> Hmm. I went in to writing that thinking "shared isn't so bad". 
>> Now I've thought about it, it is pretty damn useless. What's 
>> the point of knowing that data is shared without knowing how 
>> to safely use it? I guess it protects against completely naive 
>> usage.
>
> The real issue with "shared" is that objects may change status 
> during runtime based on the state of the program.
>
> What you really want to know is when a parameter is "local", 
> that is, guaranteed to not be accessed by another thread during 
> the execution of the function. If so you open up for 
> optimizations.

What significant optimisations does SC-DRF actually prevent?