Magento 2 What is the meaning of "please re run magento compile command"

magento setup:di:compile

magento2 How to reindex and cache flush : 

1. Need comand for re-index
     magento indexer:reindex

2.  magento 2 cache flush command

    php bin/magento cache:clean and php bin/magento cache:flush

Install the base data for reference

php bin/magento sampledata:deploy
php bin/magento setup:upgrade

Need to compile after upgrade
magento setup:di:compile