“Can you hold it steady for me?”
You're trying to change a light bulb in a light fixture that's high up. You don't have a ladder, so you're standing on a stool. You're a little worried that you'll fall, so you ask your girlfriend to hold the stool, and ask her like this.
Can you hold it steady for me?
This is a way of asking someone to help you:
Hey, can you get that for me?
Use it with people in your family, your close friends, and people who you have authority over like your employees or students.
With people you're not as close with, you can ask "Would you mind ___ing for me?":
Excuse me. Would you mind watching over my stuff for me for just a minute?
When something is "steady", it means that it's not changing a lot. For example:
They seem to have a steady relationship.
The stock market has been steady this week.
Be careful stepping on that. It's not steady.
So "holding ___ steady" means holding it so that it doesn't move around. You can hold a ladder steady, a stool that someone is standing on, a piece of wood that you're nailing, etc.