Typically you do not have the option of using bold, underline, or italics online and on MU*s. Therefore various other ways of emphasizing words have been devised. Marking a word with unusual puncutation, for example asterisks or underscores, gives it emphasis (to be read louder or more strongly).
For example: The dog ate my homework!
The *dog* ate my homework!
The dog ate _my_ homework!
The dog >ate< my homework!
Another obvious way to emphasize a word is to put it in all caps. You can easily imagine someone shouting,
"THE DOG ATE MY HOMEWORK!"
Now imagine someone who always talked that way...
"HI GUYS! HOW ARE YOU? I'M DOING GREAT. WHAT ARE WE DOING TONIGHT? ANYTHING FUN?"
On a MU* this kind of writing is used almost exclusively to indicate high volume. Speaking like this regularly will make your companions want to hold their ears. To come across politely on MUDs, reserve writing in all caps for times when you really mean to be yelling or shouting.