While it is not clear that Polonius meant the same by this phrase as I do here, it is important stuff.
"To thine own self be true."
Polonius may well have meant by this to have your actions consistent with what he considered to be good character - represent yourself well by your actions. Today, we extend this expression to include that one's actions should be consistent with one's ideals, beliefs, and thoughts - that we may know the person by their actions, deeds, and expressions.
This is so important to live. People are happier, more comfortable in their own skins, self-assured, when they behave like the people they are. This is the kind of message that we impart to children (our own, our children's friends, and in my case - also my students).
Our children should let their actions and words reflect themselves - and the best of themselves. This can be a self-check to insure that they are keeping their thoughts, minds, and deeds healthy and compassionate. If you wouldn't say it or do it in front of your grandmother - you shouldn't say it or do it.
We need, however, to be aware of what this expression means to a broad diversity of people.
I attended a meeting last week... there was a young man there who was worried. He was moving to a foreign country and he was worried about how he would be perceived. What if he wanted to date someone local, would it be legal? Could he just be himself there? Would he need to worry about being arrested? Should he lie about who he is?
As a married, middle-aged mom - I don't have tothink about this; I can give my husband a kiss in public, hold hands, and share an intimate moment on date night. But, in our ultra-religious community, if I were gay - whether I could express myself safely would be a constant theme in my life. To whom can I be honest without worrying about repercussions for myself, my children, my partner, my job...
We need, as compassionate people, to find a way to let people be true to their own selves. As long as everyone involved in any relationship is consenting (and able to consent) - no one else need worry or even think about what dynamics are involved.
If you never have - attend a PFLAG meeting, it may open your eyes.