In addition to blending colors, this VOP can apply to any vector data that you'd like to blend together.  The first vector value goes into the primary color, the second value goes into the secondary value, and the bias amount controls how much primary or secondary gets used.  A bias amount of 0 means that the primary gets used.  A bias value of 1 means that the secondary gets used.  Any value in-between is a blend between the two.

Primary Color:

--  This is where you define or plug in the first vector value.

Secondary Color:

-- This is where you define or plug in the second vector value.

Bias Amount:

--  As mentioned above, this controls the blend between the primary and secondary values.  This is usually where you plug in some sort of point attribute that selects between the primary/secondary value.

--  The adjust bias seeks to answer the question, "what happens if a bias value is less than 0 or greater than 1?" and "How quickly should the parameters blend between 0 and 1?"  By default, there will be a linear interpolation.  That means if you set the Bias to 0.28, you'll get 28% of the secondary value and the rest primary.  You can change that interpolation though if you want.

--  Clamp To Unit Range =  If values go out of range, they simply clamp to the nearest primary/secondary value.

--  Ease In/Out within unit Range =  This will remove the linear interpolation between 0 and 1.  Instead, the value switches really quickly to the other side once you pass a value of 0.5 

--  Smooth with Cardinal Spline =  This option is great if you understand mathematic lingo.  According to the docs, "If you choose the "Cardinal Spline" option, the two colors are interpolated non-linearly and the result is similar to using a Spline operator with its "u" input acting as the bias, and the primary and secondary colors piped into the first/second and respectively third/fourth "key" inputs."  In practice, I don't see much of a difference in how it blends between 0 and 1.

--  Use As Is =  If values go beyond 0 and 1, then the values will continue going in that direction.  This means that the values will not clamp once they go past 0 or 1.  They will keep going in that direction.

Color Blending Space:

--  This parameter is there to make a convenient switch over to RGB if you're plugging in HSV color data.  In practice, I almost always leave this at RGB.