How to enter a new line on a cell
Use Alt+Enter to start a new line in a cell.
When you add a line break, Excel automatically changes the cell's format to Wrap Text. But unlike normal text wrap, your manual line break forces Excel to break the text at a specific place within the text. This gives you more precise control over the appearance of the text than if you rely on automatic text wrapping.
To remove a manual line break, open the text in the cell for editing and press Delete when the insertion point is located at the end of the line that contains the manual line break. You won't see any symbol to indicate the position of the manual line break, but the text that follows it will move up when the line break is deleted.
See also this tip in French: Comment entrer une nouvelle ligne sur une cellule.