In Swift, let is used to declare a constant, while var is used to declare a variable.

Here are some key differences between let and var in Swift:

• The value of a constant cannot be changed once it is set, while the value of a variable can be modified.

• Constants are more efficient and faster to use than variables because the value of a constant does not need to be looked up at runtime.

• Constants are preferred over variables whenever possible because they can help prevent unintended modifications to the value of a piece of data.

Here is an example of how to declare a constant and a variable in Swift:

let maximumNumberOfLoginAttempts = 10 
var currentLoginAttempt = 0

In this example, maximumNumberOfLoginAttempts is a constant with a value of 10, and currentLoginAttempt is a variable with a value of 0. The value of currentLoginAttempt can be modified, but the value of maximumNumberOfLoginAttempts cannot be changed.

Here is another example of how to use let and var in Swift:

let numberOfDaysInWeek = 7
print(numberOfDaysInWeek)  // Output: 7

var currentTemperature = 22
print(currentTemperature)  // Output: 22

currentTemperature = 24
print(currentTemperature)  // Output: 24

As you can see, the value of the constant numberOfDaysInWeek cannot be changed, whereas the value of the variable currentTemperature can be modified.

It is generally a good idea to use constants whenever possible, as it helps to ensure that the value of a particular piece of data does not change unexpectedly. This can make your code easier to understand and maintain.

Hope this helps.