Adding a couple of details to Badunit's post:
A Numbers Document is a Spreadsheet. It will contain one or more Sheets, blank canvasses on which may be placed one or more Tables, text boxes, graphic objects, and/or charts (graphs). Tables contain one or more Cells. Cells may contain data that has been entered directly, or that has been calculated by a formula placed in the cell.
"If the two tables are on different sheets, the reference to a cell in a table on the other sheet will look something like Sheet 2::Table 1::B2."
The Sheet name part of the reference is needed only if the table has a name that is also the name of another table in the document. If there is no other table with the same name, Numbers does not need the sheet name, and will drop it if the user has included it.
References created by clicking on cells (as in Badunit's instruction above) will automatically construct the cell reference and include the necessary parts of the cell address.
Regards,
Barry