This word starts with お which makes it more polite. Then, there's the kanji 母 which you know means mother. Last, there's the honorific さん which further adds respect (to the mother). So, this is the nice way to say mother. You use it when you're talking to your own mom or talking about her with friends. You can also use this when you're talking about your friends' moms.

If you need to talk about someone's mother that you don't really know, or you aren't close to, you'll want to use お母様, instead. Don't worry, you'll learn that last kanji in a few lessons. Until then, it's pronounced, おかあさま. Kay?