What does CultureInfo.InvariantCulture mean?

I have a string of text like so:

var foo = "FooBar";

I want to declare a second string called bar and make this equal to first and fourth character of my first foo, so I do this like so:

var bar = foo[0].ToString() + foo[3].ToString();

This works as expected, but ReSharper is advising me to put Culture.InvariantCulture inside my brackets, so this line ends up like so:

var bar = foo[0].ToString(CultureInfo.InvariantCulture)
        + foo[3].ToString(CultureInfo.InvariantCulture);

What does this mean, and will it affect how my program runs?