When someone is crucified, they are tied or nailed to a big wooden cross and left there to die. It's a really, really mean thing to do to someone. It's like being punished with lots of pain and suffering.
Long ago, a group of people called the Romans would sometimes crucify people as a punishment. They were in charge of a big part of the world back then and they wanted to show everyone who was boss. If someone did something they didn't like or were seen as a threat to the Roman way, they might be crucified.
Being nailed to a cross would hurt a lot. It could take days for the person to die. They might have trouble breathing because their chest felt heavy, and they couldn't move their arms and legs. People who watched might throw things at them or make fun of them.
The person being crucified might feel really sad, angry, or scared. They might pray or talk to God for help. Some people might even shout and curse at God. It really depended on the person and what they believed.
Crucifixion is a sad and painful thing, and it's not something that should ever be done to anyone.