when creating a web page what does <em> and <tt> mean??

Question

or stand for?

Answer 1

is a tag that defines Emphasis. It looks the same as italics. You'd use it like this:

"This is a sentence that defines the use of the emphasis html tag"

is a tag that defines "typewriter text" or "teletype" (hence the "tt"). It results in characters that are all even-width (or "monospace").

Answer 2

means emphasis, and is usually rendered as italics.
renders text in a teletype, or fixed-width font.