"WYD" is a text message abbreviation that stands for the phrase "What are you doing?". It is typically used in casual conversation via text message.
Text message abbreviations were originally designed to decrease the number of keys an individual had to press in order to send a text message, as well as to work within the character limit of an SMS message. Modern smart phones are equipped with full QWERTY keyboards, however, and SMS messages are no longer shortened to a limited character count. This makes text abbreviations, or "txtspk," less necessary. The shortened slang lives on, though, and has been accepted in casual usage.