You say "It's all yours" when you're finished using something and want to tell the next person that they can use it:
A: Hey, are you done with that?
B: Sure, it's all yours.
You can also use this phrase when you're almost finished using something. For example, if someone in your family wants to use your computer, you can say:
Let me just do a few more things, and then it's all yours.
"It's all yours" is friendly and casual.