在element组件里面的value值里的.map是什么意思，求解

```html
<el-form-item label="建造年份">
                                                    <el-select multiple :disabled="isable"
                                                        :value="elem.BuildingYearList.map(p => p.year)"
                                                        @change="(value) => handlerYearChange(value, elem)"
                                                        placeholder="请选择" @change="changeYear(elem)">
                                                        <el-option v-for="elem in UnregisterbuildYear" :key="elem.title"
                                                            :label="elem.title" :value="elem.value">
                                                        </el-option>
                                                    </el-select>
                                                </el-form-item>

```