Be yourself means be true to who you are while still keeping the social veneer in tact.
In other words, if being yourself would mean swearing up a blue streak in public to every waiter trying to serve you, the world does not, seriously, want you to "be yourself."
Being yourself DOES mean saying you like Legos when asked if you like Legos, and not pretending you don't because the person you are talking to just said they didn't. Lol, I'm can't imagine my son ever NOT being himself in that way; the thought of pretending to have interests he does not would never cross his mind. But NT's do it ALL the time, feign interest in things they are not interested in for the sole reason of getting a date or getting a job. I still remember taking a job application test and filling in that I was a "morning person" because I thought everyone preferred morning people - - THAT is "not being yourself," and downright counterproductive.
The problem is, being AS, you may see the required social veneer as being inherently at odds with who you are. To someone who is NT, it isn't; it's just the ribbon on the box and not all that important; you just put ribbons on because you put ribbons on; it doesn't change the box.
Being yourself means that you wrap up your personality and interests in a box of the right shape and size but still put ribbons on it that will please the audience. Not being yourself is stuffing a Volvo into a ring box and trying to pretend that is the real you; not going to work, eventually the Volvo has to burst out.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).