When you left click on a cell in an Excel worksheet, it becomes enhanced with a thicker border. You will also notice that the number at the far left of the cell row, and the letter at the top of the cell column, will be shaded differently. The cell you have chosen is now the active cell, and its name or reference is the cell column letter followed by the cell row number (The number and letter that are shaded differently).

In this image, cell G4 (the one with the thick border) is the active cell. The column letter and row number of the active cell is displayed in a text field near the upper left corner of the Excel grid. If you enter a column letter and row number into this text field (a cell reference) the corresponding cell will become active.

You can enter text or a number directly into the active cell. If you click a button on the formatting toolbar, the formatting will be applied to the active cell. If there is already data in the active cell, the formatting option you choose will be applied to this data. In summary, to change the contents (formatting or data) of any individual cell, click on it to make it active, and then make changes.