The "readonly" modifier is used to indicate that a property or variable should not be modified once it has been initialized. It ensures that the value of the property remains constant throughout the lifetime of the object or variable.
 
Example with an array:

const numbers: readonly number[] = [1, 2, 3]; 
numbers[0] = 4; // Error! Cannot assign to an element of a readonly array 
numbers.push(4); // Error! Cannot push to a readonly array