The particle -은/는 can be used after any noun, which turns it into the *topic* of the sentence. It's different from the particle -이/가 which are added to nouns to turn them into a *subject*. This is a hugely important thing to know and understand and only really makes sense after some experience using them. The topic particles are used to show contrast and one really useful way to think of them is as "As for noun/name". For example: My friend went to the park yesterday. (As for me), I went to the park today. = In this case you could add -는 to 저 to make 저는, which has the meaning "As for me/I". You can see the contrast... My friend did X. As for me, I did Y. I hope that helps.