Do you mean & by itself, or as part of a thing like &? The latter is an HTML “character reference,” which is used to represent characters that clash with bits of HTML syntax. For example, HTML tags are wrapped in “angle brackets”, also known as the less-than and greater-than symbols, so if you want to write those symbols, you need to use < and > instead.
All this encoding crap is supposed to happen behind the scenes, but some software forgets to handle them correctly, and the character references “peek through” and get rendered as plain text.
Edit: this post is a good example of software being weird. It automatically converted the code for the greater than symbol into the symbol itself, but not the others.