The InstallData will be run during the module install and its used to load initial data to DB table.

The module install script will run when you run the following command line

php bin/magento setup:upgrade

File : app/code/PHPCodez/First/Setup/InstallData.php

<?php
 namespace PHPCodez\First\Setup;
 use Magento\Framework\Setup\InstallDataInterface;
 use Magento\Framework\Setup\ModuleContextInterface;
 use Magento\Framework\Setup\ModuleDataSetupInterface;
 class InstallData implements InstallDataInterface {
   public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context) {
   $setup->startSetup();
   $setup->getConnection()->query("INSERT INTO phpcodez_post SET name = 'Post 1'");
   $setup->endSetup();
 }
 }