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Apostrophes look like commas upside down and are used to help the reader understand what the writer is trying to say. There are rules on how they should be used, although today many people seem not to know or care about the rules.
One place they are used is before the end 's' in a possessive word like cat's in the cat's paws. In this case we know there is only one cat being referred to. If the writer had written the cats' paws then we would know he's referring to all cats.
Another place is in the word he's as in he's done it again. Here, the apostrophe indicates that a letter or letters have been left out. The sentence should really read he has done it again, but we shorten it and put an apostrophe in.
Plurals do not have an apostrophe. e.g potatoes has no apostrophe, unless of course you are referring to something belonging to the potato, like its skin, in which case you say the potato's skin, or in the plural potatoes' skin.
Words like its only have an apostrophe if you mean it is, rather than belonging to it.
