This is actually a two-part puzzle. The first part is the harvest itself, and is best done by keyboard alone. If one taps one of the letters in the field repeatedly (for example, the 'A' key), eventually all those letters will be harvested, and that letter will show up on the right in a different color. Tapping that key again will lead to having to harvest those letters all over. Also, if that letter on the right should disappear, those letters will return to the field.
The trick here is that once one kind of letter is harvested, one must quickly switch to another letter until that one appears on the right, and so on. A little speed and plenty of accuracy are needed, but the entire field can be harvested fairly easily. When that's done, you are left with just the letters on the right, and are now on to the second part of the puzzle.
What do you do with those letters? Make words, of course!
(HINT: The words to make here are all fruit.)