Learn Polish: On jest blisko.

Polish often drops subject pronouns like ja (I) or ty (you) because the verb already tells us who is doing the action. However, for third-person words like on (he), ona (she), or ono (it), the verb jest is exactly the same. Keeping On here makes it clear you mean "he" rather than "she" or "it".

Why do we use jest here?

Jest is the specific present tense form of the verb być (to be) that pairs with the third-person singular pronouns. You use it whenever you are saying "he is" (on jest), "she is" (ona jest), or "it is" (ono jest).

Can I use blisko to say someone is a "close" friend?

No, blisko refers to physical distance, meaning "near" or "close by". If you want to talk about a close emotional relationship, you would use a different word (the adjective bliski). In this sentence, blisko simply tells us where he is physically.

Is the Polish word On pronounced just like the English word "on"?

Not quite! The Polish o is shorter and rounder. It sounds more like the "o" in the British English pronunciation of "pot" or the vowel sound in "ought".