In JavaScript, the let keyword is used to declare variables with block scope. Variables declared with let are limited in scope to the block (enclosed within curly braces) in which they are defined, including nested blocks. Here's an explanation of how let works:

1. Block Scope:
Variables declared with let have block scope, which means they are only accessible within the block in which they are defined. This includes if statements, loops, and any other block of code. For example:

   function greet() {
     if (true) {
       let message = "Hello";
       console.log(message);
     }
     console.log(message); // Throws ReferenceError: message is not defined
   }

   greet(); // Output: Hello

In this example, the message variable is declared with let inside the if block. It is accessible within the block and can be used for logging. However, trying to access the variable outside the block will result in a ReferenceError since it is not defined in that scope.

2. Block-level Declarations:
Variables declared with let are not hoisted to the top of their scope like variables declared with var. They are only accessible after they have been declared. For example:

   console.log(name); // Throws ReferenceError: name is not defined
   let name = "John";
   console.log(name); // Output: John

In this case, attempting to access the name variable before it is declared will result in a ReferenceError. The variable needs to be declared before it can be used.

3. Shadowing:
Variables declared with let can "shadow" variables with the same name in outer scopes. This means that the inner variable takes precedence over the outer variable within its block. For example:

   let count = 5;

   if (true) {
     let count = 10;
     console.log(count); // Output: 10
   }

   console.log(count); // Output: 5

In this example, there are two count variables, one declared in the outer scope and another declared in the if block. The inner count variable shadows the outer count variable within the block, so the value of count logged inside the block is 10. Outside the block, the outer count variable retains its value of 5.

4. Redeclaration:
Unlike variables declared with var, variables declared with let cannot be redeclared in the same scope. Attempting to do so will result in a SyntaxError. For example:

   let count = 5;
   let count = 10; // Throws SyntaxError: Identifier 'count' has already been declared

In this case, the second declaration of count using let causes a SyntaxError because the variable has already been declared in the same scope.

The let keyword introduced block scoping in JavaScript, providing more predictable scoping behavior and reducing the chances of accidental variable redeclaration and hoisting-related issues. It is generally recommended to use let instead of var for declaring variables within block-level scopes.