Originally, it usually means money that you lend or use to someone/something.(principal) It has other meanings, but in common conversation, they aren't used. There are some idioms using it. If you say '본전도 못 찾다', it means you can't even get the principal. So it's not worth doing. If you use '밑져야 본전', it means that the worst thing to happen is to keep the principal. So it's similar with 'give it a try.' So in common situation, 본전 not only means the principal but the things you should keep basically.(that belongs to you.)