What are azeotropic mixtures?
If you have two components in a mixture, say water and ethanol, and you heat the liquid to the boiling point of that mixture (at the given pressure), you may find that the composition of the resulting vapor and the remaining liquid are different. In that case, you have a non-azeotropic mixture and the composition of vapor and liquid will change as you continue boiling.

For the case of water/ethanol, at some point, the composition of vapor and liquid will match (and no longer change, no matter how long you boil, until all liquid has evaporated). That composition/mixture is called an azeotrope.

The liquid mixture of oxygen/nitrogen is an example for a system that does not show an azeotrope.