Implicit and explicit conversion

You want to show all data for the employee with id 3. The problem is that you don't remember the datatype of the EmployeeId field. You decide to run two queries:

SELECT *
FROM Employee
WHERE EmployeeId = '3'

and

SELECT *
FROM Employee
WHERE EmployeeId = 3

You can paste them to the console on the right to see the output. Notice that both queries run without errors. Which type of conversion is responsible for this?