When you're logged in under your user account, Rewordify.com remembers all the words you learn. Once you learn them, the site no longer rewordifies it (in most cases). This helps the site to grow with you, only helping you with words that you don't know.
Here are some details about how this works.
Some words that you learn might also appear in phrases, idiomatic expressions, etc. If an individual word that you learned appears in a phrase that is also being rewordified, the phrase is still rewordified.
For example, if you learned the word sovereign, the site won't rewordify that word anymore. But, it will still rewordify the expression sovereign state, until you learn that phrase.
If you know a word that's rewordified, you can do two things: A) Click on it and click + add to my words, and then do a Learning Session with it. That will prevent that word from being rewordified again, or B) Increase the rewordifying level (click the Settings link at the top of the page) so that the difficulty threshold is increased.