Final answer:
AutoFill is the method used in Excel to automatically fill in missing cells based on a pattern in a series, such as extending a sequence of months or numbers.
Step-by-step explanation:
The method applied in Excel that allows for automatically filling in the missing cells below is called AutoFill. AutoFill is a feature that enables users to extend a series of numbers, dates, or even text based on a pattern determined by the initial selection. For example, if you have a cell with 'January' and you pull down the fill handle (a small square at the bottom-right of the selection), Excel can automatically fill the subsequent cells with the following months, 'February', 'March', etc.