Conditional Formatting in Microsoft Excel, LibreOffice Calc, and some other spreadsheet programs that allows the user to automatically add color, text styles, and so on to certain cells or ranges of cells based on the data in the spreadsheet.
Conditional formatting is useful to automatically format large ranges of cell, particularly based on the value of the cell or its neighbors. For example, you could automatically format cells with a negative value as red. However, conditional formatting is much more powerful - it can also let you color-code cells based on (for example) how close they are to the average value of all cells, or their relationship with other cells. Another use well suited for conditional formatting is to set a cell's background and text color to white based on some condition, thus hiding the data in cells based on some condition (for example, hiding cells on a bank ledger sheet if they are the same as the last date's value).
In both Microsoft Excel and LibreOffice Calc, conditional formatting is activated by going to Format>Conditional Formatting on the main menu bar.