A polite/respectful way to refer to one's mother (or someone else's), commonly used in conversation.
お母さん、今日の夕飯は何にする？
Mom, what should we have for dinner today?
お母さんはいつも優しくて、頼りになる存在だ。
My mother is always kind and reliable.
週末はお母さんと買い物に行った。
I went shopping with my mother on the weekend.
In some contexts, it can be used politely or familiarly to refer to one's wife.
彼女のお母さん（＝妻）は料理が得意だ。
Her wife is good at cooking.
家に帰るとお母さんが待っている（ここでは妻を指す場合がある）。
When I get home my wife is waiting (in contexts where it refers to wife).
Used familiarly to address an older woman as 'you', or sometimes to mean 'she' in certain contexts; a colloquial/dialectal usage.
お母さん、もう行くよ！（近所のおばさんに対して親しげに言う）
Hey ma'am, I'm leaving now! (said familiarly to an older woman)
あの人？ああ、お母さんは元気にしているよ。（『彼女』の意味で使う場合）
That person? Oh, she's doing well.