What is a "bias value" of floating-point numbers?

b0lt has already explained how bias works. At a guess, perhaps you'd like to know why they use a bias representation here, even though virtually all modern computers use two's complement essentially everywhere else (and even machines that don't use two's complement, use one's complement or sign-magnitude, not bias).

One of the goals of the IEEE floating point standards was that you could treat the bits of a floating point number as a (signed) integer of the same size, and if you compared them that way, the values will sort into the same order as the floating point numbers they represented.

If you used a twos-complement representation for the exponent, a small positive number (i.e., with a negative exponent) would look like a very large integer because the second MSB would be set. By using a bias representation instead, you don't run into that -- a smaller exponent in the floating point number always looks like a smaller integer.

FWIW, this is also why floating point numbers are typically arranged with the sign first, then the exponent, and finally the significand in the least significant bits--this way, you can take positive floating point numbers, treat those bits as integers, and sort them. When you do so, the result will have the floating point numbers in the correct order. For example:

#include <vector>
#include <algorithm>
#include <iostream>

int main() { 
    // some arbitrary floating point values
    std::vector<double> vals = { 1e21, 1, 2.2, 2, 123, 1.1, 0.0001, 3, 17 };
    std::vector<long long> ivals;

    // Take those floating point values, and treat the bits as integers:
    for (auto &&v : vals) 
        ivals.push_back(*reinterpret_cast<long long *>(&v));

    // Sort them as integers:
    std::sort(ivals.begin(), ivals.end());

    // Print out both the integers and the floating point value those bits represent:
    for (auto &&i : ivals) 
        std::cout << i << "\t(" << *reinterpret_cast<double *>(&i) << ")\n";
}

When we run this, the result looks like this:

4547007122018943789     (0.0001)
4607182418800017408     (1)
4607632778762754458     (1.1)
4611686018427387904     (2)
4612136378390124954     (2.2)
4613937818241073152     (3)
4625478292286210048     (17)
4638355772470722560     (123)
4921056587992461136     (1e+21)

As you can see, even though we sorted them as integers, the floating point numbers that those bits represent also come out in the correct order.

This does have limitations with respect to floating point numbers. While all (non-ancient) computers agree on the representation of positive numbers, there are three representations that have (fairly recently) been used for signed numbers: signed magnitude, one's complement, and two's complement.

Just treating the bits as an integer and comparing will work fine on a computer that uses signed magnitude representation for integers. For computers that use one's complement or two's complement, negative numbers will sort in inverted order. Since this is still a simple rule, it's pretty easy to write code that works with it. If we change the sort call above to something like this:

std::sort(ivals.begin(), ivals.end(),
    [](auto a, auto b) { if (a < 0.0 && b < 0.0) return b < a; return a < b; }
);

...it will then correctly sort both positive and negative numbers. E.g., input of:

std::vector<double> vals = { 1e21, 1, 2.2, 2, 123, 1.1, 0.0001, 3, 17, -0.001, -0.00101, -1e22 };

Will produce a result of:

-4287162073302051438    (-1e+22)
-4661071411077222194    (-0.00101)
-4661117527937406468    (-0.001)
4547007122018943789     (0.0001)
4607182418800017408     (1)
4607632778762754458     (1.1)
4611686018427387904     (2)
4612136378390124954     (2.2)
4613937818241073152     (3)
4625478292286210048     (17)
4638355772470722560     (123)
4921056587992461136     (1e+21)

In single precision floating point, you get 8 bits in which to store the exponent. Instead of storing it as a signed two's complement number, it was decided that it'd be easier to just add 127 to the exponent (since the lowest it could be in 8 bit signed is -127) and just store it as an unsigned number. If the stored value is greater than the bias, that means the value of the exponent is positive, if it's lower than the bias, it's negative, if it's equal, it's zero.