All of us have different distances between the two eyes. The distance between the centers of pupils of both eyes is called as pupillary distance (PD) or inter-pupillary distance (IPD). This distance is important when you want to use glasses for sight. The power of the lens in the glasses is not equally distributed. It is usually more in the center and lesser in the periphery of the lens. Therefore, the alignment of the center of the eye with the centre of the lens is important for us to see clearly with the prescribed lenses.
If you want to order your glasses online, you will need to inform them about your pupillary distance (PD). Here is a simple method to find out your PD. You will need an iPhone and a credit card. You will need to install a free app called PD Measure by GlassifyMe. The credit card is used for its size. Here is a video which explains how to measure the PD: