When you reply all, it goes out to everyone on the To line and the CC line. Reply only goes to the sender. When we send out a mail to many people, you might see a sentence that says 'little 'r' me'. This means to use the small R instead of the Big R. That basically means to reply only to me instead of replying to everyone on the mail thread.

A mail is also referred to as a thread. That is because in the office, a mail can go on back and forth among many people so it becomes a chain of mail that started from one email. I'll use these terminologies in the next several example sentences.

"I sent out a mail to the whole group. I requested that they little 'r' me with their ideas."
"When I receive a mail sent out to multiple people, I keep forgetting to send to all."

This next sentence is not a spoken sentence. It's a common sentence used in email when someone forgets to send to everyone.

'Resending to include everyone.'

If you see this, that means the person sent a reply to the sender instead of replying all.

"Start a mail thread on your idea. I think everyone should get in on this discussion."
"Do you want me to start a mail thread for this?"