The Legend question mark at the bottom right of the table opens a help panel that shows what you can edit, how to work with several rows and which keyboard shortcuts are available. The panel appears automatically the first time you open a new table.
What the help panel contains
Can be edited: Select the cell and type. Green means changed but not saved yet. Red is invalid and blocks saving, and the reason is shown at the top. Yellow with a person icon is someone else's change, and the banner under the row lets you choose their version or yours.
Working with several rows: Select the rows in the column on the left. Select cells with Shift-click, by dragging or by clicking a heading.
Keyboard shortcuts: Ctrl+C and Ctrl+V copy and paste, also to and from Excel. Ctrl+Z and Ctrl+Y undo and redo.
Cannot be edited: Muted text. Click for an explanation.
Good to know
If you close the panel in one table, it closes in all tables.
You can open it again at any time with the Legend question mark.
On Mac, use Cmd instead of Ctrl.