There’s an old joke about how to summarize every Jewish holiday: They tried to kill us. We overcame. Let’s eat.
That’s not actually an accurate description of every holiday, but it’s pretty accurate for Hanukkah (And Passover/Pesach. And Purim.) We talk about fighting oppression, and winning by a bare thread. Then we eat some symbolic food.
The particulars of the Hanukkah story are along the lines of: The Greek empire was antisemitic and banned Judaism. They tried to make all the Jews assimilate. They destroyed the Temple (in ancient times, the Israelites had a centralized worship system centered around a Temple in Jerusalem; this is no longer the case and hasn’t been for centuries) and sacrificed pigs in it to desecrate it. Then the Maccabees revolted. They won and got the Temple back. They wanted to rededicate it, and they needed to light the Temple’s light. Unfortunately, there was only enough ritually pure oil to last one day (and it takes several days to make more). They lighted the lamp anyway, and the oil miraculously lasted eight days, long enough to make more pure oil.
Lighting the menorah for Hanukkah is considered a way of publicizing the miracle. Jews light one candle to symbolize each night, and another candle just to be a candle. Some people focus on the miracle as the oil lasting longer than it naturally should have. Others focus on the miracle as surviving and maintaining Jewish culture in the face of oppression.
(Digression about the word “menorah”: The original meaning of the word “menorah” was a seven-branched lamp in the Temple in Jerusalem. That’s the thing the Maccabees needed to light. In Modern Hebrew, “menorah” usually just means lamp, and it never refers to the nine-branch thing Jews light on Hanukkah. The Modern Hebrew word for that is hanukiah. In English, “menorah” means the Hanukkah thing (except when people are translating the Bible or something). Some people might try to tell you that menorah is an incorrect word. They’re right in Hebrew, but wrong in English. In English, “menorah” is a correct word.)
There is also a custom of eating fried foods because the miracle involved oil. Two particularly popular foods to use for this are latkes and donuts.
tl;dr Hanukkah is about celebrating Jewish physical and cultural survival in the face of oppression. It’s also about an ancient miracle involving oil. Two things people do to celebrate Hanukkah are light candles and eat fried foods such as latkes and donuts.