Where does the value of the VUE variable come from?

Question

Vue.js

I can’t understand where the value of the tab variable comes from here

<button v-for="tab in tabs" v-bind:class="['tab-button', { active: currentTab === tab }]" v-on:click="currentTab = tab">

https://jsfiddle.net/morskaya_ulitochka/y3jaLe9d/

Answer 1

from v-for=" tab in tabs"