Somewhere on the surface of the glass, there is a spot of shiny metal - postage stamp size, more or less - that looks like the glass is mirrored. It's a very reactive metal, which will absorb any gas inside the tube, and will turn white when it does so if there is more than a trace of gas - i.e. even a very tiny leak. It "gets" the gas out of the vacuum, so it's called a getter.
One of the ways that an old tube might misbehave, when first put in service after long disuse, is the gradual leakage of traces of gas from the internal components. A few tens of hours at operating temperature will restore the vacuum by heat-activating the getter. But of there's a lot of gas, and the getter is white, it can't be fixed.
