When you click this button, it populates the three fields with the Width, Height and Resolution of the image you're working on.
There is only ONE reason to use this button, and it is a very rare one. You use it if you want to crop another image to the exact same size as the one you're presently viewing. You press that button to get the specs of the current image, then switch to the other image, and crop it. Both images will then be exactly the same height, width and resolution.
That hardly seems worthy of a blog post? You're dead right. It's such an obscure function that it's barely worth mentioning. The reason I'm writing this post is because an alarming number of people misuse the Front Image button.
This invariably comes up when somebody, somewhere, asks the question "How can I crop my image and keep it the same shape as the original?". Somebody else chimes in with "Just press the Front Image button then crop".
Before I discuss why that answer is wrong and dangerous, I want to stress that the question is also flawed. As I sit here, I can't think of a single reason why you'd want to crop your photo with no other purpose than "keeping it at the same shape as the original". Cropping must only be done with a specific purpose in mind. If you haven't already done so, please read my cropping tutorial.
But even if you could find a legitimate reason for cropping to the original shape, Front Image is a dangerous way to do it. Why? Because you're not only cropping, you're also re-enlarging (resampling) the remaining pixels. This is unnecessary, and detrimental to image quality.
DON'T DO IT.