Headings can contain more than one row. In the picture, you see a table that has a heading that contains two rows. Notice that most of the table contains four columns, but the first row contains only two columns. That's because two cells were merged to create a heading that spans two columns.
You can merge any number of cells — they don't need to be heading cells or even be in the same row. They can be any number of adjacent cells anywhere in your table.
Keep in mind, however, that because a table is an organizing tool, simplicity is important. Merge cells only for small alterations.
To see an example of merging two cells, click the Play button above the picture. Don't worry about remembering all of these steps now. You'll get the chance to do it in the practice.