Just like breathing, but not as skillfully, we take in prana energy into the body, sometimes in rhythm with our breath. It's similar to breathing, and if it were to completely stop, it would be a life-threatening situation. However, people live because they possess prana energy, and everyone is taking in prana, even unintentionally.
In yoga, this energy is called prana. In China, it's called "qi," or sometimes "chi" or "lun." When we intentionally take in this energy, I feel that the body becomes more deeply activated.
When you're not meditating, you don't really feel it much. But when you meditate to a certain extent and reach a state of stillness, you can start to feel the flow of energy. In that state, when you take a deep breath, you can feel prana being taken into the body.
This may vary from person to person and place to place, but in cities, it seems that prana doesn't easily enter. However, it's not like you'll die from it, so there's not much to worry about. Basically, you take in prana from the place where you live.
Prana can be efficiently taken in through food and yoga's pranayama (breathing exercises), which allows you to live a healthy life. In addition to that, you also take in the energy, prana, that exists in a place.
In my case, even when I try to take in prana by taking deep breaths, it comes in very slowly. I don't know how it is for others, so I don't know if this is normal or if I'm not taking in enough, but it feels like I'm taking it in little by little. It's like taking in the prana around you, not just through the lungs, but through various parts of the body.
The speed is quite slow, perhaps around 3cm or 5cm per second. When I consciously gather it, it moves a little, then stops, so I repeat the process of focusing my attention and gathering it further.
Sometimes, you hear stories about saints who live on prana and offerings (prasada). If we could do that more efficiently, perhaps we could live on prana alone. I'm not there yet.