Following the Prologue (Genesis 1:1-2:3), and the first major section (Genesis 2:4-4:26) we get the first genealogical interlude in Genesis 5:1-6:8. All five of the major sections of Genesis will be divided by such interludes.
For the most part, they are mere bridges, tying each major story (Adam, Noah, Abraham, Jacob, and Joseph) into the timeline of history.
It is tempting to scan these genealogies and move on, and perhaps surprisingly that is exactly the best way to approach these passages. Because the alternative temptation—to scour through these texts seeking special insights or precise details about unanswered questions—produces mountainous problems from scantest uses of ink.
In chapter five we get a quick glimpse of the first ten generations of humanity. Any attempts to calculate spans of time without exact numbers (manuscripts vary) or an insight as to what was being counted is an exercise in futility, and that isn’t even the point of this chapter. We are simply meant to get from Adam to Noah with an understanding that these are real people, not characters in fiction.