Answer:
Be true to yourself
Step-by-step explanation:
This sentence by Polonius is continued with:
'And it must follow, as the night the day, Thou canst not then be false to any man.'
This is important because it shows the (moral) effect of being true to yourself on others. In other words, if you live your life in a true and honest way, your social life will be likewise.
There is another side to these lines: it implies that you should also be true to your own caracter or personality, which is the essence of you as a person. This has led to many phylosophical discussion and questions such as:
Should I be true to myself when I hurt somebody else?
But I am afraid the answer to this question is not an easy on...