Oxygen is far more prominent at sea level which means you have a set amount of red blood cells in your body. Red blood cells are the cells in your blood that carries oxygen around the body to different tissue. Altitude training is preferred for longer distance athletes like the 5,000m and 10,000m but can be used for other sports like cycling. For altitude training to make any difference you need to be training at preferably 2,400m for a week or more.
At altitude the air is a lot thinner meaning that there is less oxygen present. So when your body is at altitude, to compensate for the lack of oxygen, your body produces more red blood cells to carry the oxygen around the body.
This is useful because when you go back down to sea level, your body will have more red blood cells than everyone else in it but you will also have the same amount of oxygen intake that you normally would. This therefore means that you would get more oxygen going to your muscles faster when you are running, swimming or cycling giving you a physical advantage. The only thing is that the extra red blood cells only lasts for around 10-20 days.