I have iphone se 2nd gen. What is the button on left side above up and down volume buttons?

That would be the mute switch.

Use the side, Home, and other buttons on your iPhone - Apple Support

terue wrote:

What is the button for on the left side above up and down volume buttons?

Ring-silent; the mute switch.

Click on the following blue-text link to more info ➡️ Use the side, Home, and other buttons on your iPhone - Apple Support

Blue text used here in the Apple forums is a clickable link to more information.