Honesty, that you've learned from it, and that it won't happen again.
How to say it
1Be honest and brief. Use neutral words like “it wasn't the right fit” or “I was let go”.
2Take your share of responsibility, without running yourself down.
3Say what you learned and what you do differently now, with proof if you can.
For example
“I was let go from my last job after a few months. I'd taken on more shifts than I could manage and I was late too often. It taught me to be realistic about my hours. Since then I've volunteered every Saturday for six months and haven't missed a single shift.”
Use your own words and your own story. This is only a shape to follow.