The delete keyword is used to reset the values of some data type and complex types such as arrays and structs. For example, if delete is used on int/uint, its value will be set to 0, which is the default value of the data type:

uint a = 1;
function reset() public {
    delete a; //resets the value of a = 0
 }

In the same way, delete can also be used to reset the array types, where it assigns a dynamic array of length 0 if the array is a dynamic array. If an array is a static array, it resets all elements and keeps the length of the array same.

When the delete keyword is used for a struct, it resets all of the variables of the struct to their respective default values.

For mappings, you can reset a value by calling delete on its key. One thing to notice here is that delete a behaves like an assignment to the a variable; it restores the default value of the type.