This is an example of a completed Skyscraper Sudoku. Imagine there is a Skyscraper on each cell in the grid, the value tells us how many floors any given Skyscraper has. The clues round the outside tell us how many Skyscrapers are visible from that side of the grid.

Looking at the first column, starting from the top and working downwards, we have,

6. Visible
8. Visible
7. Not visible (hidden behind the '8')
2. Not visible
4. Not visible
9. Visible (we can stop here)

This gives us the clue of 3 at the top of the column.