Using the "Quick Fill Square", Google Sheets makes it easy to quickly fill in the same data up or down columns and across rows.
For example, if you have sorted your Transactions sheet by the Description column, you can add the same Category to multiple transactions without having to manually enter the categories one by one. It's great for updating data for transactions you manually entered or whenever you need to change a consistent value across a large data set (e.g. gave an account a nickname from the Tiller Console and need to update historical records).
In the example below, you want to add the "Auto and Gas" category to all the Chevron and Costco Gas transactions.
Select the "Auto and Gas" cell.
Click and hold on the small square blue box in the lower right corner of the cell. This is the Quick Fill Square.
The cursor turns into a + sign.
While continuing to hold the cursor down, drag the mouse down to all the cells you want to fill.
The Quick Fill Square also works on dates. Clicking on a date and dragging to fill will enter the next consecutive date in each cell. If you want to fill with the same date, hold down the option key.
For more information see the Google Help Docs.