To separate the different gases from air, they are to be liquefied first to implement fractional distillation. The air is cooled and compressed to obtain liquid air. One can see that the boiling points (b.p.) of various constituents differ by less than 25°C.
For instance, the b.p. of oxygen is -183°C whereas the b.p. of nitrogen is -196°C. So simple distillation would convert both into gaseous state, and one would fail to separate them. Due to this reason, fractional distillation is used.
In this case, both liquids get converted into vapour state and passes through the fractionating column. The less volatile component gets condensed and becomes a liquid again. The more volatile component suffers less condensation. As a result, the more volatile component tends to reach the top of the fractionating column. This vapour at the top is condensed and collected in a beaker.
For the separation to be effective, this process is done multiple times.
