Let's say a friend comes to you and wants you to make a banner for them. They want it to say:
Happy Birthday Tom
They say they want Tom's name to be bold. So in your handwritten notes, you mark the beginning and end of the bold parts:
Happy Birthday (start bold)Tom(end bold)
Similarly, in HTML, the start and the end tags simply help the computer understand exactly where things start and end.
Next: Fundamental HTML Tags