You are working in an Excel file and entering text in a column but you want to start a new line and hitting enter/return just brings you to another cell. To edit the cell, press F2, type your text and then press Alt-Enter to bring the cursor to the next line and continue typing. The contents of your cell are now split onto two lines.
Forcing a line break is not the same as wrapping text. In Excel if you want your text to wrap within a cell, you use the Align text function.