This was already answered, of course, but I thought I'd also answer with the little bit of information I know!
So, Korean sentences tend to be written in a Subject-Object-Verb or Subject-Adjective style.
Example: I (subject) food (object) eat (verb). or I (subject) beautiful (adjective).
To indicate which words are the subject or object of the sentence, you add a particle! 가/이 and 은/는 can both be used to identify the subject of the sentence. The difference between the two is that 은/는 often has the role of comparing two things. For instance:
고양이는 집 뒤에 있어요 = The cat is behind the house.
As you can see this sentence uses the 는 particle, attached to the subject of the sentence, which is cat. This can mean that the CAT is behind the house, in comparison with something that is not behind the house. Like, say you were also talking about the location of a dog. This sentence would be saying that the cat is behind the house, but the dog is not. I hope that makes sense!
고양이가 집 뒤에 있어요 = The cat is behind the house, using the particle 가. No comparison. It's just stating the fact that the cat is behind the house!
So
날씨가 좋아요 = The weather is good.
날씨는 좋아요. = The WEATHER is good, but that's all.
I hope that all of this made sense! :) Good luck with your studies! 화이팅!