Alumna is the feminine form of "alumnus," meaning someone who graduated from a school. An alumna is a female graduate.
This word sometimes means a graduate of either gender, but it's most commonly a woman who graduates from any school — whether grammar school, high school, college, university, or med school. Schools are often proud of a famous alumna, and there might be pictures on the wall to honor them. As an alumna, you might go back for school reunions. When you're talking about a group of graduates, the word is alumni, which usually means men and women.