LT stands for <em>Less Than.</em>
Suppose you need to show the character &LT; in a web page.
You can't just type it, because the browser will think
it's the beginning of a tag.
So instead you represent &LT; by the four characters "&amp;LT;",
which the browser will convert to the &LT; you want. (There are lots
of other special characters you can get in the same way.)