I can't understand interpolation of string

Answer 1

Try to read the below link, example 7 in basic output explains string interpolation.
https://www.programiz.com/swift-programming/basic-input-output

Answer 2

This is a fancy name for what is actually a very simple thing: combining variables and constants inside a string.
var name = "John Martin"
var age = 24
"My name is \(name), and age is \(age)"