In a fashionistic sense, it means to stray away from newspaper and capitalistic definitions of what it means to be perfect and acceptable in society. It is a DIY and anti-big-company based ideology.
This is not to be confused with the "hello kitty says ACAB" monster energy gun people, who promote the ideal that more money spent=the more alt you are, which is the direct opposite definition of alt.
People who are against the promotion of perfectionism in the media, and like to look a different way than the societal norm can be considered alt/alternative.