Function keys, buttons, and menus enable you to indicate font changes, special characters, etc. HTML requires you to type that information manually. Whenever a viewer sees those two keys, it assumes that they contain a tag. If the viewer does not understand the tag, it will ignore it. Tags never appear on the screen, whether the viewer understands them or not.
Open tags are a one-shot deal, a single tag indicating a single action.
Closed tags are far more common and come in pairs. One tag tells the viewer when to begin an action, the other tells it when to stop. Tags can be upper or lower case. They can come one after another, or they can appear on separate lines. The viewer will not show the tags, and will ignore any extra hard returns that you insert.