Let & Const

"Let" and "Const" are ways of declaring variables in JavaScript and hence are also used in React JS, which is a JavaScript library.

'Let' is used when you want to declare a variable that may change in value. For instance, you might use 'let' to initiate a counter variable in a loop that changes each time the loop runs.

Example:
```
let counter = 0;
for(let i=0; i<10; i++) {
counter += i;
}
```
In this snippet, "counter" and "i" are declared with 'let' because their values are expected to change as each loop iteration progresses.

On the other hand, 'Const' is used when you want to declare a variable that will not change in value - it is constant. If you try to reassign a value to a constant variable, JavaScript will throw an error.

Example:
```
const PI = 3.14159;
PI = 3; // This will throw an error
```
In the above code, "PI" is declared as a constant with a value of 3.14159. Any attempt to reassign a value to "PI" will result in an error.

'Let' and 'const' have block scope, which means they only exist within the block they are declared in, unlike 'var' which has a function scope.

These are fundamental concepts in JavaScript and bear similarity in React JS as well.