For those whose lives have not yet included arguments about the use of the Oxford comma, it's the comma in a list after the word 'and' thus, for instance: What one needs to make coffee: ground coffee beans, water, water heater, and jug/coffeepot.
I nearly added mug/cup, but of course one doesn't need a mug or cup to make coffee
ffinnochio it is as anno says: a comma between two quite separate sentences, where either a full stop or a semi-colon should be. Yes, it matters, because it can change the sense. I had a fierce English teacher (i.e. teacher of English) at school in Scotland who made us REWRITE an essay if we used 'so' as a conjunction, or the verb 'to get'. I can't do it to this day, 50 years later.
Oscar Wilde reckoned that once, when struggling to compose a poem, he spent the whole morning deciding to take a comma out and the whole afternoon deciding to put it back in again.
On the other hand, George Orwell thought that grammar and punctuation didn't matter all that much so long as what you wrote was lucid.
I think good punctuation can improve lucidity, and bad punctuation can make things harder to understand, or make them more open to amibguity. Ambiguity isn't always a bad thing of course; I'm thinking of literary appreciation exercises that depended at least in part on the connotations of the words used.