Expletive is a grown-up word that is used when someone says something that is not very nice or polite. Sometimes people use words like "swear words" or "curse words" to show anger, frustration or to express strong emotions.
When you hear "expletive deleted" it means that someone has removed the bad word or phrase from what was being written or spoken. This is often done in official documents, news articles or TV shows to make sure that the message is appropriate for everyone.
Imagine you drew a picture and then realized that you made a mistake. You might want to erase that part of the picture and start over again. "Expletive deleted" is a way of erasing the bad word or phrase that someone said or wrote so that the rest of the message can be understood easily without causing any problems.