Imagine if you were walking around a city and needed to use a phone, but couldn't find one.
Now imagine if you saw a bright red box that looked like a big phone booth! You could walk inside and use a phone that was inside the box.
That's what a red telephone box is - it's a big box that people used to use to make phone calls. They were mostly in the United Kingdom and were really popular in the olden days before everyone had smartphones.
People liked the red telephone box because it was easy to spot and they knew they could use the phone inside if they needed to. Today, there aren't as many around because most people have phones in their pockets, but some people still like to keep them as a symbol of the past.