Opposites are things which are completely different from each other and don't share similar qualities. For example: up and down, left and right, on and off, responsible and irresponsible, happy and sad, shy and outgoing, etc. It's important for you to learn about opposites because English speakers write and talk about things in comparison and contrast (as opposites!) all the time.
You can use opposites in general descriptions, to tell a story, to make jokes, etc. Here are some examples so you can practice using opposites.
1. When we first met, your hair used to be short. What happened? It's so long now!
2. Is the light on or off?
3. Quit acting so irresponsibly; you need to be more mature!
4. I'm thinking about dinner...are you hungry or full?
5. Is your new boyfriend tall or short?
6. Oh, you had 3 tests today? What a lucky day!
7. Well would you rather be in the office working or on a beach on vacation?
8. Did they accept or decline your offer?
9. Is your new job easy or hard?
Getting the hang of it? Great! I knew you would!