The popup allows you to provide more user interaction to your extension.
{
"name": "Extension Starter",
...
"browser_action": {
"default_icon": {
"16": "images/icon16.png",
"24": "images/icon24.png",
"32": "images/icon32.png"
},
"default_title": "Extension Starter",
"default_popup": "popup.html"
},
...
}