vue项目中$t()的意思是什么

<el-form-item
:label="$t('staffManage.modal.userName')"
prop="userName"
class="is-required"
>
     <el-input
     v-model="formValue.userName"
     :placeholder="$t('staffManage.modal.placeholder')"
     />
 </el-form-item>

export default {
      modal: {
        placeholder: '请输入',
        userName: '姓名'
    }
  }
}

import staffManage from './platform/staff-manage/index';
const locale = {
    staffManage 
}
export default locale;