Not quite for a five year old but: These states have agreed with each other to allocate their votes in the electoral college to the candidate who wins the national popular vote. The agreement only goes into affect once states controlling 270 electoral college votes have signed on. This is the amount required to win the presidency, so at that point the popular vote winner will necessarily win the election.
It's likely that this agreement would face challenges in court if/when it eventually goes into effect.