Place tents to some empty cells so that tents don't touch each other even diagonally. Pair each tent with a tree adjacent horizontally or vertically. This should be a 1 to 1 relation. The clues outside the grid indicate the number of tents in the corresponding row or column.
Grid control: Click the mouse button at the selected cell to add/remove a tent. Additionally you can connect tents and tree with lines which don't affect correctness check. More details about grid control are at the help page.