element-ui的回调参数什么意思？

上面这个是什么意思呢？
还有下面我碰到的问题：

希望有人可以帮助我解答！

今天又测试了一遍，pager写成其他的也对，不知道昨天是怎么测的，竟然错了，谢谢

<template>
  <div class="block">
    <span class="demonstration">显示总数</span>
    <el-pagination
      @size-change="handleSizeChange"
      @current-change="handleCurrentChange"
      :current-page.sync="currentPage1"
      :page-size="100"
      layout="total, prev, pager, next"
      :total="1000">
    </el-pagination>
  </div>
  <div class="block">
    <span class="demonstration">调整每页显示条数</span>
    <el-pagination
      @size-change="handleSizeChange"
      @current-change="handleCurrentChange"
      :current-page.sync="currentPage2"
      :page-sizes="[100, 200, 300, 400]"
      :page-size="100"
      layout="sizes, prev, pager, next"
      :total="1000">
    </el-pagination>
  </div>
  <div class="block">
    <span class="demonstration">直接前往</span>
    <el-pagination
      @size-change="handleSizeChange"
      @current-change="handleCurrentChange"
      :current-page.sync="currentPage3"
      :page-size="100"
      layout="prev, pager, next, jumper"
      :total="1000">
    </el-pagination>
  </div>
  <div class="block">
    <span class="demonstration">完整功能</span>
    <el-pagination
      @size-change="handleSizeChange"
      @current-change="handleCurrentChange"
      :current-page="currentPage4"
      :page-sizes="[100, 200, 300, 400]"
      :page-size="100"
      layout="total, sizes, prev, pager, next, jumper"
      :total="400">
    </el-pagination>
  </div>
</template>
<script>
  export default {
    methods: {
      handleSizeChange(val) {
        console.log(`每页 ${val} 条`);
      },
      handleCurrentChange(val) {
        console.log(`当前页: ${val}`);
      }
    },
    data() {
      return {
        currentPage1: 5,
        currentPage2: 5,
        currentPage3: 5,
        currentPage4: 4
      };
    }
  }
</script>

字面意思，就是事件回调函数里面的参数。