A getter is the splash of material, generally evaporated metal, deposited on the inside surface of the glass of a vacuum tube, typically at the top, sometimes on the sides and/or bottom. They are composed of materials that have a strong affinity for gas evolved within a tube during its lifetime (or left over after after sealing due to imperfect vacuum). Generally, the getter absorbs or binds gas atoms up into itself, where hopefully they will remain inactive during the life of the tube.
If a tube is massively overheated, the getter can be revaporized and disappear from the surface of the glass. OTOH, their normal action tends to be improved when the tube is heated from normal operation, as opposed to when its at room temp..
If the vacuum is breached, e.g., the glass cracks, typical getters will quickly (although not instantly) oxidize to chalky white powder. Any such tube is completely finished.
Sometimes you'll find a tube with no apparent getter left at all and it still works well! Longer term prospects are gloomy though. Also, some tubes have the gettering way at the bottom behind the base, and it's easy to underestimate that there is much left.
The getter tends to change color and gradually disappear, become patchy, develop changes on the edge, etc. over the life of the tube, so appearance of getter is one clue about history of service and possible life left in the tube. But this can be much less than an exact science, for various reasons. It's also worth noting that the appearance of getters in NOS tubes can vary considerably, for various reasons, so it's not always clear what the appearance/area was at baseline . . .
Technically, the getter is not the various metal support structures ("halos", "D-shaped", "bent square", flying saucer", etc.) that hold the getter metal in place until it is flashed inside the sealed, evacuated tube by a high frequency pulse during manufacture, although you'll commonly see references to "halo getters", etc. Properly, the "getter" is the "flashing".
There's actually quite a lot of information available on the internet about getters, and a fair amount of that is even true!