One is the knowledge of the Self which is our true nature. And the other kind of knowledge is that which is used like soap.
Do you know why it is said to use knowledge as a soap? So that you do not become arrogant about having knowledge. You should not think, ‘I know so much; I know everything.' You should not become egoistic about knowledge. So to get rid of such an ego, it is said that knowledge should be used like a soap.
This is the actual meaning.