Ghost rows are rows that have not been initiated yet. They are mostly used when discussing System data types. A row is said to be "initiated" when there is at least one value in a cell in that row.
For instance, in the animation below we insert a new column B and set it to the Autonumber data type. Once we select Add, Spreadsheet.com automatically adds autonumbers to those rows with data in them (rows 3-7) beneath the Table header row. The rows below row 7 are said to be "Ghost rows". We can see them, but there's nothing in them and they haven't been initialized so the system doesn't insert autonumbers for them. However, as soon as we add a "New task" to row 8, Spreadsheet.com recognizes row 8 as initialized and assigned B8 an autonumber.
See our Introduction to Rows article for a general overview of rows.