Normally when you send an e-mail, you just send it to "one" person, the same way that you send a letter on paper through the post office, to just one person. But with e-mail you can send the same letter to a number of different people. If each person who gets the e-mail, sends their reply to everyone who got the first e-mail, you can have a group discussion with ordinary e-mail.
This can be a handy way to organize something like a family reunion or family get together, or just for family members to keep in contact with each other. It's like a conference by e-mail.
You probably already know that to send an e-mail to one person, you have to tell the computer "who" you want to send the e-mail to.
In most programs, you can also send the same e-mail to a number of people, by telling the computer all the people you want to send "To" (i.e. you can put a whole bunch of people in the "To" field of one e-mail, and it will send that e-mail to each of them).
When you get one of those e-mails that has "multiple recipients," if you click on "Reply to All," your reply will go to everyone that got the original e-mail. If everyone that receives the e-mail uses the "Reply to All" command, you can have a private e-mail discussion group (and you don't need anything else to do it, just your normal e-mail program).