Sometimes in text messages people say "XD". What does XD mean?

XD is actually an emoticon/emoji.

Much like how you read =D as an expression of great joy or excitement, XD is to be read and understood in a much similar way, though often the individual using XD is enjoying great humor.

The expression used is one of laughter or excitement with both eyes closed, like someone is laughing very hard and trying to catch their breath.

Emoji: XD = "X" = eyes = "D" = the mouth. Means you are laughing means you are trying to catch your breath. You can't stop laughing. LOL!