Okay, picture this: Imagine a bunch of kids playing on the playground at school. One of the kids is always talking big and acting tough, but they never actually get into any fights themselves. Instead, they try to convince other kids to fight for them. That kid is called a "chickenhawk."
In politics, a chickenhawk is someone who talks a big game about military action and warfare, but they themselves have never served in the military or have any real experience with fighting. They basically want to send other people (usually young soldiers) to fight in their place.
People often see chickenhawks as cowardly and hypocritical, because they talk tough but don't have the courage to actually serve in the military themselves. It's kind of like that kid on the playground who talks smack but never actually gets into a fight.