Constants and variables are used to store and manage data in your program in Swift. Constants are values that cannot be changed, while variables are values that can be changed.

Constants are declared using the let keyword, followed by the name of the constant and its type. Here is an example of how you could declare a constant in Swift:

let pi = 3.14

In this example, pi is the name of the constant, 3.14 is its value, and Double is its type. Because pi is declared using the let keyword, its value cannot be changed once it is set.

Variables, on the other hand, are declared using the var keyword, and their values can be changed at any time. Here is an example of how you could declare a variable in Swift:

var score = 0

In this example, score is the name of the variable, 0 is its initial value, and Int is its type. Because score is declared using the var keyword, its value can be changed throughout the course of the program. For example, you could increase the value of score by 10 using the following code:

score += 10

In general, you should use constants whenever possible, because they help make your code more predictable and easier to understand. Constants also make it easier to optimize your code, because the compiler can make certain assumptions about the values of constants that it cannot make about variables. However, there are times when you need to use variables, such as when the value of a particular piece of data needs to change during the execution of your program. In those cases, you can use variables to store and manage that data.