Deyr fé, deyja frændr,
deyr sjalfr it sama,
ek veit einn, at aldrei deyr:
dómr um dauðan hvern.
Cattle die, | and kinsmen die,
And so one dies one's self;
But a noble name | will never die,
If good renown one gets.
-Havamal, (The Sayings of the High One, verse 77)
"ek veit einn" is often translated as "one's reputation", backed up by the second half of verse 78, which reads:
svá er auðr sem augabragð,
hann er valtastr vina.
One thing now | that never dies,
The fame of a dead man's deeds.
but I think this is misrepresentative of the real meaning of this verse. The concept of mortality, not merely for men, but for all things, even the gods, is a core theme within old germanic thought. This is shown most famously with Ragnarok, the apocalyptic event wherein the entire world and almost all the gods will be destroyed. But even beyond this the germanic gods were unusually mortal - not only could they be killed, and were even fated to die, but they aged, too, and only though mystic means were they able to keep their youth.
A good reputation, and especially things like glory and fame, is earned by deeds, but dependent on men. But the verse clearly shows, men die too, and without men there can be no glory in any traditional sense - who would tell the tales? Who would there be to be famous among?
I think, rather, it has to do with the concept of time in old germanic thought. The book "The Well and the Tree" goes into more detail on this than I can here, but essentially, the past builds up on itself sort of like layers of sediment, becoming more and more "solid" and entrenched the more layers are added on top of that layer, on into infinity. What this means is that any deed or action you take, and just as importantly, all the deeds and actions you DON'T take, will never be "lost" from the perspective of the universe, because the way it settles into the vast chronological strata of the past continues to support and affect the future, even a hundred trillion years from now. The universe is shaped partially because of those deeds, but it is also NOT shaped in other ways because other potential deeds you might have done were not taken.
Of course this is all cognate and related to the concept of Brahman in Hindu thought and Indo-European beliefs, and even the concept of mass-energy and potential energy in modern science and the actual structure of the universe far beyond that, but that's the gist of it.