Q:

What does !default in a css property value mean?

The twitter bootstrap code has a lot of CSS properties with a !default at the end.
E.g.
p {
  color: white !default;
}

What does !default do?
UPDATE
My bad for not being clear. I am using the SASS part of Bootstrap.

A:

!default is used often in Bootstrap Sass. It is similar to a reverse !important. All of Bootstraps Variables are set using !default to allow the developer to further customize bootstrap. With !default sass will only define a variable if it has not already been set.
This allows for more flexibility. 
//Example1 Dress color = red
$auroras-dress-color: blue;
$auroras-dress-color: red;

//Example2 Dress color = red
$auroras-dress-color: blue !default;
$auroras-dress-color: red;

//Example3 Dress color = blue
$auroras-dress-color: blue;
$auroras-dress-color: red !default;

So Why is this important? 
Bootstrap is a package. Most people don't edit the Bootstrap source. NEVER UPDATE THE BOOTSTRAP SOURCE. To customize bootstrap you will add your own variable file and compile it with the bootstrap code but never touch the native bootstrap package. Bootstrap sass's page has the full skinny on how to customize and compile it in the documentations.
I don't know why less does not do this. I have not worked much with less and do not know if it has it's own built in variable management. 
Example fiddle
https://jsfiddle.net/siggysid/344dnnwz/

A:

Twitter Bootstrap uses LESS as far as I've seen. On the other hand, !default is actually part of Sass, and is used for giving Sass variables ($var) default values, which would make it invalid in your given context, even in Sass.
Besides, I've not been able to find any references to !default in the LESS documentation, and to my knowledge it is exclusive to Sass. Are you sure you found this in Bootstrap's source and not elsewhere? Because I honestly don't remember seeing Sass/SCSS code in Bootstrap's stylesheets.
For what it's worth, the only valid token that starts with ! in CSS is !important, which you may already be aware of.

