Type the romaji syllable corresponding to the displayed kana. The equivalencies can be seen by scrolling down the page. If you type the correct syllable, the kana will be refreshed and the input box will be cleared. Typing '1' will also refresh the kana and clear the input box. Typing at least 4 characters will clear the input box, but not refresh the kana.
You can select what kana to view. The dropdown menu allows you to practice only katakana, only hiragana, or a mix of both. You can also have the equivalent katakana and hiragana displayed together. The checkbox allows you to turn the addition of dakuten on and off. There are 48 syllables without dakuten and 30 with dakuten, so turning the checkbox on will display up to 78 unique syllables. Modifying either of these options will refresh the kana and clear the input box.
You will need the japanese Unicode character set on your computer in order to see the kana symbols properly. Otherwise, they may appear as boxes with numbers in them.