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A spreadsheet is a grid. Columns run down the page and are named with letters: A, B, C. Rows run across and are numbered: 1, 2, 3.
Each box in the grid is a cell, and it's named by its column and then its row. So C4 is the cell in column C, row 4. The box above the grid, called the Name Box, shows the name of the cell you've selected.