"The weather is like that."
그렇다 is the infinitive or dictionary form (stem 그렇~ ).
그래요 is its final transformation to a present tense, polite level but irregular verb.
The stem can be the words for "this / that / that (over there)" as part of the verbs meaning.
if you are learning from the mobile apps, please go the website and see the "Tips and Notes" for "Polite" speech lessons. For all lessons, knowing those tips makes the lessons easier to understand. Sometimes, I wish I was a sponge, or at least a brain that soaks up these tips not just to store but to understand!