This is a dynamic Bonus Goal, as indicated by the green two-arrow symbol, meaning that it changes from turn to turn. You must submit a word of at least the indicated length. Once you do, the goal will change to a different length word. Do this the indicated number of times to achieve the Bonus Goal. Example:

You must make a word of at least 5 letters. When you do, the goal changes:

Now you must make a word of at least 6 letters. Continue until you have met the goal a total of 4 times over the course of the level.

Sometimes the required length will go up as the goal changes, and sometimes it will go down.