Watch the video above to understand how to use 는 (neun) and 은 (eun).
When it comes to usage, you can think of the English phrase as for.
오늘은 바빠요. (오늘=today;바빠요=busy)
As for today, I'm busy.
는/은 can be used to show contrast:
오늘은 바쁜데, 내일은 안 바빠요. (내일=tomorrow;안 바빠요=not busy)
I am busy today, but I'm not busy tomorrow.
는/은 is used more for answers.
Q: 이름이 뭐에요? (What is your name?)
A: 제 이름은 말로입니다. (My name is Mallo)
For a more detailed explanation as to when you can use this particle, check out this article from Tammy Korean!