What is the difference between `let` and `var` in Swift? When to use which?

Hello everyone! I just started learning Swift, and I have a fundamental question that seems to haunt all beginners. I can't fully understand the difference between `let` and `var`. I read that `let` is a constant and `var` is a variable. But in practice, I'm running into confusion. For example, I wrote this code: ```swift var myName = "Alexey" myName = "Ivan" print(myName) // Prints "Ivan" — makes sense, it's a variable ``` But then I tried the same with `let`: ```swift let myAge = 25 myAge = 30 // Error: Cannot assign to value: 'myAge' is a 'let' constant ``` This is logical — the compiler complains. But then I read that `let` can be undefined until first use. I tried to do this: ```swift let userName: String // Some code...