The meaning of you have my back is To be willing and prepared to help or defend someone.
If someone says I have your back then they mean that they are willing and are prepared to help you in a time of need or defend you when needed.
If you say you have someone's back then it means that you're willing and are prepared to help that person or defend that person when needed.
Other ways to say have your back are.
I'm on your side.
I'll back you up.
I've got you covered.
I'm with you.
I got you.
The definition of have your back.
To be willing and prepared to help or defend someone; to look out for someone in case they need assistance.
Example sentences of have my back or have your back include.
Don't worry about those thugs, buddy. I've got your back if they ever come bother you again.
You can always rely on your parents to have your back.