hydrogen is a combustible gas but it does not support combustion. what does this mean?

It means hydrogen burns itself but does not allow substances to burn in it. Pure hydrogen burns with oxygen with non-luminous blue flame with unique pop sound. The reaction is exothermic. The ability of hydrogen to produce heat with oxygen makes it to act as a good fuel. A mixture of hydrogen and oxygen on ignition produces explosion. This shows that hydrogen is a combustible gas.

 Let us see the nature of hydrogen which does not allow substances to burn in it.

Consider a jar filled with hydrogen with inverted mouth downwards and introduce a burning candle near the mouth of jar, the gas burns at the mouth of the jar while the burning candle gets put off. This shows that hydrogen is combustible but not a supporter of combustion.