Emails deleted from my MacBook don't delete from my iPhone

If I delete an email from my iPhone, it is automatically deleted from my MacBook. Yay! But if I delete an email from my MacBook, it is still on my iPhone. How do I get emails I delete from my MacBook to automatically delete from my iPhone? And PLEASE tell me like I am a complete idiot. I've read some other queries along this line, and I can't understand any of the answers that bandy about terms such as "IMAP" or "synchronized folders". (BTW - this was not a problem until my iPhone automagically updated a week or so ago. Why Apple felt the need to mess up my email is perplexing.)