"Ma'am" is a title that people use to address a woman. It's the female equivalent of "sir".
This word is used differently in different English-speaking regions. Throughout most of the U.S., employees at banks, airports, post offices, and so on call female customers "ma'am" as a sign of respect. Students also traditionally call their female teachers "ma'am".
In the southern parts of the U.S., "ma'am" is even more common. Some men call any older woman that they don't know "ma'am".
"Ma'am" was originally short for "madam", but people don't really think about that when they use this word in modern times.