A Core Timer Service is a function that will periodically execute at a nearly exact interval of time.
In most microcontrollers, this type of function is called a timer interrupt.
If you are unfamiliar with interrupts, they are a special type of function that is only called when an external condition (such as an input signal on an I/O pin) is met.
For instance, an external interrupt could be setup to be called every time it detects a LOW to HIGH transition on an I/O pin.
In the case of the Core Timer