What does XD mean?

It's a shorthand for a smiley--supposedly a laughing smiley. The X is the person's eyes, which are squinting shut with mirth, and the D is the open and laughing mouth.

https://www.macmillandictionary.com/us/dictionary/american/xd

XD also represents the following:

XD Christmas Day
XD Mixed Doubles (sport)
XD ex dividend
XD Executive Development
XD Cross Dock (distribution)
XD Execute Disable Bit (Intel)
xD Extreme Digital (flash memory card developed by FujiFilm and Olympus)
XD Extra Dimension
XD Dies Laughing
XD Extreme Duty (Springfield Armory line of handguns)
XD Without Dividend
XD Exploratory Development
XD Exothermic Dispersion
XD Xtreme Duelists (Yugioh Clan)

http://www.acronymfinder.com/af-query.asp?Acronym=xd&string=exact

490. (When translated from later Roman numerals...)

[Editor's note: see below.]

XD can't mean 490. The only proper way to show 490 in Roman numerals is CDXC.

"Don't subtract a letter from another letter more than ten times greater. This means that you can only subtract I from V or X, and X from L or C, so MIM is illegal." Illegal! Mercy me! That's from the above link.

Put more plainly,
"X and L can only modify up to a C. For example, 490 is not written XD. First you resolve 400 as CD and then you resolve 90 as XC. Put them together and 490 = CDXC."

Whether or not the old Romans consistently adhered to such rules is another question, but present-day sticklers do.

XD is a symbol to describe eyes and laughing mouth. Mostly when you see something funny.

Read more here:
https://linguaholic.com/linguablog/the-meaning-of-xd/