Help with breeder box, what does this do?

What does this partition do?

 

It is a 'v' slot so when the momma fish gives birth the babies fall through so she can't eat them, then you remove her and keep the babies in there (or a breeder's net) unlike they are large enough not to get eaten by the adults.

 

Something, I can tell you that.

Lol we replied at the exact same time XD

 

The breeder box is so small, will the mom Platy be stressed since there isn't a lot of space to swim in?

 

Yes, most fish stress in smaller breeder boxes and sometimes will abort their babies. Unless you see her going into labor I would not keep her in there, it is barely any space.
If you get some floating plants (real or fake works) most of the fry will hide in it and survive, instead of stressing out your momma platy.

 

What kind of floating plants are there?

 

Water wisteria can float, frogbit works. Those two are probably what you're looking for since you can pop them in quickly.

Anacharis can work, but they're cold water plants. They can adapt to warmer water but it takes a while. They're good for short term use since they're sold in large bunches. Just beware of snails!

 

The easiest would probably be a fake plant taken out of its little plastic holding, one of the fluffier looking ones. You can go with hornwort, anacharis, cabomba, or water sprite for live plants (only need one, anacharis is one of the easiest live plants to get most of the time).