As you reach a higher level, you'll be studying octaves. What are octaves? After you've done your scales, after you've done thirds and sixths, you have what is called octaves.
Octaves are the same note, but eight notes away. So, if you have an A here, you have the same A on the G string, but an octave lower. Same goes for B. Those are octaves. Now, octaves are played together and they're difficult because you have to play them so in tune. So, here is an example of how to practice octaves at home. What I tell my students is, you practice them separate, separate and then together. Separate, separate, together. And you use full bows with a nice big forte sound.
That was separate, separate, together. And you can go up another octave as well. So, that's how, you're listening to the notes and then you're playing them together and that's how you get them perfectly in tune.
And those are octaves.