Up to ten rows of each sheet can be designated as the column header. This is called freezing rows. Every sheet has a horizontal guideline which separates header rows from the rest of the sheet. This guideline has a gray handle. The gray handle initially appears above row “1” on the left hand side of the sheet. To set the number of rows in the column header, drag the handle up or down using the mouse.
Or you can set the number of rows using the menu option View → Freeze Rows.
There is also a vertical guideline which works in a very similar manner, and lets you freeze as many as five columns as row labels.
When you create a blank document, it comes with an initial sheet. This sheet does not have a column header.¹ Additional sheets, if you create them, are the same: they do not have column headers to begin with.
The Android app imposes a limitation on this feature. While it does show frozen row and column headers properly, it does not let you control which rows or columns are frozen. You need to use a PC based browser to change your row and column headers.
