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Share your podcast's web address with friends, and they can subscribe to your podcast.

Hi,
Is "to" in the above optional? If not, what does it mean? Thanks.
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no. 'Subscribe to' is a phrasal verb, it means to sign up to recieve something which is regularly published or broadcast, like a magazine, newspaper, or podcast.
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Old Man Gordonno. 'Subscribe to' is a phrasal verb, it means to sign up to recieve something which is regularly published or broadcast, like a magazine, newspaper, or podcast.

Thanks, Gordon.

But what does "to" in question mean? Or what is it similar to? Is it similar to the "to" in "I gave a book to John?"
Well, I have to say that in a phrasal verb, the preposition itself doesn't have 'meaning', because the whole phrase has a unique meaning in combination.

But your question about similarity is good...I think it's similar to 'listen to.' I listened to the radio.

'Give to' is different, because give can take an indirect object. You can also say, "I gave John a book". The function of to in that sentence has a clearer meaning.