A blood feud is when two groups or families get into a fight that lasts a really long time. It's called a "blood feud" because the fight often involves people getting hurt or even killed, and that can make the families angry and upset with each other for generations.
It's like if you and your friend had a fight, and you got mad and hit them. Then, they got mad and hit you back. You both might be upset with each other for a while, but eventually, you would probably make up and be friends again. But in a blood feud, the fight goes on and on, with each side trying to get revenge on the other.
Sometimes, blood feuds start because of something that happened a long time ago, like a family member being killed or someone being accused of stealing. Even though everyone involved might have been long gone, their families can still be angry and want revenge.
In some places, blood feuds are still happening today, but they are not as common as they used to be. Laws and police officers help to prevent fights from turning into long-running feuds, and people are realizing that getting along and forgiving each other is a better way to live than being angry and fighting all the time.