What does the auto fill feature do?

Answer 1

auto-fill on Web Browsers works by remembering what you have previously typed into the search boxes, and automatically filling the search box with your selection.

auto-correct (aka predictive text) on mobile phones uses a dictionary that is programmed into the phone and, based on what you've already typed, make a prediction as to what word your intending to type.

Google auto-complete (unbelievingly) works by anonymously sending what you type to Google HQ where a Google auto completer makes a prediction as to what your going to type, types it out and it goes back to your screen.

Answer 2

It guesses the rule that defines the sequence of values or formulas in the selected range, and extends the sequence into the following cells as you expand the range by dragging the lower right corner of the range with your mouse.

Answer 3

It enables you to automatically fill out common lists, such as days of the week. If you put the word Sunday into a cell, and then drag down from it, it will put in the rest of the days of the week. It will do the same for months. It will take the pattern you use too. So if you type Jan for January, Autofill will put Feb in next and then Mar and so on. These options for special lists are built in, but it is also possible to set up your own lists.