"Know Thyself" is an ancient Greek saying which is very popular in the spiritual circles as well as in the generic life. This can have various implications depending on the place and time at which is being said and its holds true for all circumstances.Talking even in purely worldly terms a person needs to know himself/herself in a way that one should know one's strengths and weaknesses so that appropriate measures can be taken which helps one to advance in the desired direction in life.
This philosophy of knowing oneself (Thyself) also leads us to a previously mentioned point namely the "I" factor. Of course one can only know oneself if the "I" is known.
Since metaphysics is a separate world in itself having bit different set of rules than the common world, the words can be bit misleading here.
For example the knowledge of "I" does not simply mean to be aware of one's existence which everyone is surely aware of, but refers to a deep realization which cannot be properly expressed in words but can only be reached through processes of deep contemplation or religious meditation under the guidance of an able teacher or master so to speak.