Everything has vibrations, which means, also our thoughts and energies. I would define the Aura as a electro-photonic vibration response of an object to some external excitation (such as an ambient light for example).
As we know, aura consists of colors. The if it's all just colors, why can't we easily see the aura? Let me explain.
Our eyes have 3 photosensitive cells: Red, Green and blue (RBG).
In the nature and the world around us, our eyes catch many colors which make the same mixture of Red, Green and blue, even if the spectra (the vibration of light) might be different.
For example, think of the color green. It can be made by:
-The combination of yellow and blue.
-Total green resonance.
Our eyes will not see the distinguish between those two. So, our eyes are not sharp enough to catch the spectrum (vibration) of originator colors.
The reason can be searched in the retina of our eyes. None of our eyes are fully healthy in the retina. Various lights (like computer screen, TV and etc) damage our retina, where we contain those 3 photosensitive cells. The reason why children are much more capable of seeing or "sensing" energy in the aura is because their retina isn't damaged yet. (As people believe children can see spirits better. It's not a matter of belief 'whether you believe in spirits or not', but it's a scientific fact)
In order to see aura with physical eyes, we need to train our eyes for a sharper view by making a stronger connection between two hemispheres of the brain and by making our eyes see beyond the wavelength of 0.3 and 0.7, which is the length that our eyes catch as light. We can do it by concentrating on one spot and gathering all our senses in one little point.
Seeing your own aura will not be as easy as it may sound. Our eyes, more or less, possess the ability to see the aura but they need training, same as psychic abilities.
I already shared a method of the astral sighting of the aura. Here's another one. Go in front of a mirror (better to have a white background) and stare at your third eye (the chakra between your eye brows), keep yourself focused and resist the temptation to look around. Keep your eyes there. You may fail to see any colors at the first time. That's fine as your eyes are just starting to warm up. Keep trying some days until you finally notice a color beyond just white.
When you have successfully done it, practice viewing someone elses aura as well. Remember, no one was perfect at the beginning but practice makes perfect.