Concatenation means, simply, to join one string to another. You can think of it as "adding" the strings together. When a program concatenates two strings, the result is a single string value.
let cat = "Cat"; let dog = "Dog"; print(cat + dog); /* CatDog would be printed. Notice there is no space between the two strings. */
To add a space between "Cat" and "Dog", you need to concatenate a string containing a space to the expression.
print(cat + " " + dog); //The string: 'Cat Dog' would be printed.
You can also concatenate numerical values - even if they aren't of type string - which can be very helpful!
A simple example:
let x = "COMP"; let y = 110; let result = x + y; print(result); /* Output: COMP110 Note that the type result is of type string */