It means be yourself and don't worry what others think. Dr Seus contains a lot of play on words.

Because those who judge you don't matter and those who matter don't judge you

آنهایی که میفهمند اهمیت نمی دهند آنهایی که اهمیت میدهند نمیفهمند!!!!!!!!!!!!!!11

thank all , these was helpful enough

be who you are: be yourself. and say what you feel: say what you want, say what you believe. because those who mind don't matter: because those who care about us we don't need them. and those who matter don't mind: and those who we need and we care about them, don't care about us. (hope that will help you cause it is just an approximate meaning)