because of its high repeatability, it is now packaged into a component, Baidu

Sub-component:

<template>
    <div>
        <div class="group">
            <label>{{title}}</label>
            <input type="text" placeholder="" @input="changeData()" v-model="val">
        </div>
    </div>
</template>

<script>
export default {
    props:["title"],
    data () {
        return {
            val:""
        }
    },
    methods:{
        changeData:function(){
            console.log(111);
            this.$emit("input",this.val);
        }
    }
}
</script>

parent component:

<cmsGroup title="" v-model="username"></cmsGroup>

what does the subcomponent this.$emit ("input",this.val); mean? the official website says: this.$emit (event, value) but my parent component doesn"t use events, and the effect comes out. I don"t understand what the "input"" really is here