"asdf" is the sequence of letters that appear on the first four keys on the home row of a QWERTY or QWERTZ keyboard. They are often used as a sample or test case or as random, meaningless nonsense. It is also a common learning tool for keyboard classes, since all four keys are located on Home row. asdf is also typed in lowercase letters most of the time.
'asdf' is a default password for some systems. Therefore the word is used in many dictionary crackers, since there is always the possibility that some user has left their password as 'asdf' by default.
The Internet has taken to the idea that many users type in the characters 'asdf' when they're not searching for anything in particular and just want to fill in a given field. If you run a search through any given search engine, you'll see many websites listed with the keyword 'asdf'.
Many internet users who are short for time simply type in 'asdf' as their username and password to obtain quick access to the information or links that have been restricted. This has led to many websites specifically invalidating email@example.com as an email address. Many websites and forums now require signing-up or registering to access a page's information or to have access to a download link. Similarly, some online games have restricted using asdf or asdfg as a password, as it is so easy to guess.