Thank you for the opportunity to help you with your question!
Hi, according to business, a contra entry is a Book keeping entry that is entered on the opposite side of an earlier entry to cancel its effect on the account balance. It is recorded to reverse or offset an entry on the other side of an account.
Example: Check this tutorial http://www.accountingcapital.com/other-topics/contra-entry/
Please let me know if you need any clarification. I'm always happy to answer your questions.