An ideal gas is a theoretical gas composed of non-interacting point particles. This implies that individual particles have zero volume and that intermolecular forces amongst them are negligible.
Real gases (like oxygen in the air) have non-zero volume and intermolecular forces. Nonetheless, at low pressures they often behave like ideal gases.
Under low pressure, the distance between gas particles is very large relative to the volume of the gas. In addition, the intermolecular forces become negligible due to the separation of particles.
The ideal gas concept is a therefore a good approximation for gases under at low pressures.
This is important because it is much easier to calculate the volume, pressure and temperature of an ideal gas (as compared to a real gas).