If you look at the hair anywhere on your body you will see that it doesn’t stick straight out. This is because its evolutionary function was to trap air to keep us warm. Now we have invented clothes and a carbon based economy we don’t need this functionality so much.
You hairs lie in a constant direction, this is called the grain, and that direction is different all over your body and it differs from person to person. What interests us, obviously, is the face and once again hairs grow in different directions in different parts of your face. Just take a good look at someone with a full beard to see the extent of this.
So, what has this got to do with shaving? Well, there is a huge difference to what happens when you shave with the grain (WTG) across the grain (XTG) or against the grain (ATG), so you need to know which way the grain goes on different parts of your face. To find this out you need to take the diagram above and then give your face a feel when you have grown a good stubble, it is really easy to discern the grain, so you can then put little arrows in the zones on the diagram. Once you have done this you are armed with the knowledge to shave better.
I have said repeatedly that shaving is personal to you. Do what you like and what works, ignore everyone else. Some people think you should only shave WTG and this is fine, it almost certainly causes the least irritation. Some people think you should do three passes: WTG, XTG then ATG, which they, probably quite rightly, think gives them the closest shave. If you want you can experiment and mix and match.The important thing is to have the knowledge with which to experiment.
I tend to mostly shave WTG but when I rinse between passes I can feel where needs the extra cutting, these places then receive extra XTG and even ATG attention to get them under control.
Of course some people knowingly just ignore the whole grain thing altogether. One perfectly viable technique is to do one pass downwards then one pass upwards, it works for some.