Windows: Press Alt + Enter while editing the cell
Mac: Press Control + Option + Return while editing the cell
Double-click the cell or press F2, place the cursor where you want the line break, then use the shortcut
Enable Wrap Text if you want the line break to display properly
In formulas, use CHAR(10) for a line break in Windows
In formulas on Mac, use CHAR(10) as well, then enable Wrap Text