You somehow was misled by the example.
In fact it´s entirely possible to write (and to find elsewhere) the sentence "review words that
you have learned."
Thus, in our case here the ommission of the relative pronoun "THAT" is possible because it is the very " object of the relative clause
In this case the process is similar to what happens with pronouns! They can replace the noun/the subject as well is of that that I am talking about, please don´t say I claimed that they can be left out...!
Summing up, in "review words that you have learned.":
THAT refers to WORDS; but then you had used WORDS then, so THAT could be left out with no loss of meaning.http://esl.fis.edu/grammar/rules/relative.htm
As a rule of thumb, when we leave out the relative the "that" (here in our example) and the sentence don´t get "weird", when we understand it just the same, then we can ommit the relative pronoun. I mean, with time we get used to it, we do it even without thinking of the grammar note.
But that comes with time, the experience, for the time being stick to the rules.