Vue.js: What is middleware in Nuxt 3?

1// middleware/auth.ts
2export default defineNuxtRouteMiddleware((to, from) => {
3    const auth = useAuth()
4    
5    if (to.meta.requiresAuth && !auth.value) {
6        return navigateTo("/login")
7    }
8})

1<script setup>
2definePageMeta({
3    middleware: "auth"
4})
5</script>