The Try button is very useful for hard Sudoku problems. If you get to a point where you can't make progress without trying one of, say, two options in a cell, click the Try button. Then try one of the options. If it turns out to be right and you solve the puzzle, then that's it. But if you reach a broken position, click the button again (which is now called Untry). It will restore the position to where you were before you tried the failed option. Now you can try the second option.
The puzzles are computer generated. Sometimes an easy puzzle may be more difficult than it should be and vice-versa. There is exactly one solution to a puzzle.
The puzzles published on Saturdays and Sundays are very difficult and will often only be solvable by the very best human solvers.
The source code for this user interface as well as the puzzle generating and solving code are FLOSS and available on Github.