She's going to create a conclusion based on that evidence. From that moment on, the likelihood is that someone will not like what she does. She concludes that Francis was born somewhere around 1855. It's really hard to guess much closer when you are dealing with census data, due to rounding and lying. In other words, it's an estimate, and a really good estimate compared to, say, guessing from looking at a photo.
Nobody likes to type the same thing over and over.
Neither does Annie. She wants to copy the formula for the birth year down to the next two cells. The user can drag that fill handle to adjacent cells and copy the formula. It's a short cut version of copy and paste, and works the same way. When Annie drags the fill handle for Francis' birth year down to cells C4 and C5, she sees the years 3 and 15 for Mathilda and Joseph, respectively.