Learn Spanish (Latin America): Unas correas tuyas están en la jaula.

In Spanish, you cannot put an indefinite word like unas (some) and a short possessive like tus (your) before the same noun. To say "some of your leashes" or "some leashes of yours", you must put the possessive after the noun using its long form, which is tuyas in this case.

Why does tuyas end in -as? Doesn't the ending depend on the person who owns it?

The ending actually depends on the thing that is owned, not the owner! Because correas is feminine and plural, the possessive adjective must match it, becoming tuyas. It belongs to "you" (singular), but the word itself changes to match the leashes.

I sometimes mix up ser and estar. Why do we use están in this sentence?

We use están because the sentence is talking about the physical location of the leashes (in the cage). Whenever you talk about where something or someone is located, you always use estar, even if the object belongs to you.

I've seen the word correa used for other things. Does it only mean "leash"?

Correa is a versatile word that can also mean "belt", "strap", or even a "watchband". However, because the sentence specifically mentions a cage (jaula), "leashes" is the most logical translation here.