Table Inspector pane
Down at the bottom of the Table Inspector pane is a checkbox that controls what happens by default when you hit the Return key as you are entering text in a cell. That box is checked in the basic template, so hitting Return takes you out of the cell. Un-checking that box will cause Return to give you a new line rather than moving the focus to a different cell.
Where do I find the Table Inspector pane?
iMac 27", macOS 10.13