Autobiographical memories are the memories that provide us with a sense of self.
So while we might remember mathematical rules that allow us to do algebra, and we might remember that the United States was officially founded in 1776, those are examples of semantic memories, which are treated differently by our brains—those are more about objective knowledge, rather than self-knowledge.
That said, semantic memories are often intertwined with episodic memories—memories of personal experiences and feelings from various times and places—and that combination creates autobiographical memories: remembrances of us experiencing things, in context.
Interestingly, there's (robust) evidence that from about age 30, onward, we tend to remember more about our lives from ages 10 through 30 with more clarity and consistency.
So although we've got a huge number of autobiographical memories tucked away in our brains, some of those memories, from distinct periods in our lives, seem to arise more freely and with more detail after a certain point.
It's suspected this period—remembered so clearly because of what's often called the "reminiscence bump"—is especially potent to our memories because that late-adolescence to early-adulthood age-range is a period of massive growth (physical, neurological, social, etc) and a time of abundant change and recalibration.
There's a lot going on, basically, and that's true both internally and externally. And consequently, we soak up more data during this period, and can thus recall things with greater clarity and ease—because all that data is there, overshadowing other, less data-rich periods.
The opposite can also (seemingly) be true during periods of our lives that are less novelty-rich.
Recent (less robust) research suggests, for instance, that people may be reporting feeling like the past several years—which were largely defined by a global pandemic—have seemed to just rush by because they have fewer compelling, colorful, memorable moments from this period.
If this theory is correct, many of our COVID-19 pandemic experiences seem flat and unmemorable not because time moved any differently or because our brains were shut down, but rather because many of us were locked away in various ways, and our days were thus less defined by new-ness and surprise.
We were limited to the same environments, saw the same people, did the same things over and over and over again, and that tends to push our brains into a sort of autopilot, during which they collect less data, which then leads to less well-defined, and less easy-to-summon memories.
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