Just for some further info on what Specular F0 means in the PBR rendering system. It is controlling the IOR ( index of reflection )
This simply means the rate of reflection at specific angles to the surface. As @nomadking pointed out previously in cg this had many names , specular , mirror , gloss etc. But they were not very accurate and bad approximations that did not factor in light absorption.
In PBR and other modern rendering systems there is only one term to define this and that is reflection. All material types have a specific real world IOR that defines their reflection. OF coarse reflection is also affected by micro surface shapes, this makes a reflection be blurred or sharp. This is defined as roughness ( or gloss ) there is no official standard for this in the rendering world the one is just the inverted state of the other and you will notice sketchfab supports both inversions.
Getting back to IOR , you can google it and will find heaps of information of specific IOR values for every different type of matter you get.
matter can be divided into two categories , dielectric ( non metallic ) and metals. You will be surprised here to see that many dielectric materials all fall within a close range of each other in regards to IOR. But to explain it visually lets use a sphere. For most Dielectrics the IOR will cause less reflection at angles close to the view angle and become stronger when approaching the tangent angle , on a sphere that would be the very edges. So the center is weaker and edges stronger. For pure metals The reflection strength is pretty uniform regardless of the angle. Think like a chrome ball.
You can even test this for yourself on sketchfab an observe the results. Notice that if you set the metal slider to full , changing the Specular F0 slider has no effect. This is because of what I mentioned above. So the specular F0 is useful only for dielectrics.
You can use the slider without a texture and it will apply the same value to every pixel of the material or you use a linear black and white texture to control the IOR strength. This is so you can do materials like a painted surface with parts of the paint chapped etc..
Lastly the last thing to know about light with regards to dielectrics and metals is that metal surfaces tint the reflected light while dielectrics ( for the most part ) just reflect the light without affecting the reflection color.