could someone please explain this statement in 11th ncert textbook :

liquid in glass thermometers show different readings for temperatures other than the fixed points because of differering expansion properties.  a thermometer that uses a gas however gives the same reading regardless of which gas is used. experiments show that all gases at low densities exibhit same expansion properties............

also what is absolute zero and triple point

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Actually the statement means that the liquid in glass thermometer shall show different readings,with different liquids,due to differences in various properties like Viscosity and Coefficient of Thermal Expansion.

While on the other hand,in gas thermometers we try using the properties of expansion of gases,to find the temperature and which overall is same for all gases(assuming they are ideal).V is directly prop.to T and hence u can say that gases(ideal Gases) expand on increase of temperature.

what are ideal gases ................