An improvised firearm is a homemade gun that someone has made out of ordinary objects they can find around them. It may not look like a real gun and it may not have the same kind of power as a purchased gun. You should never try to make one at home because it can be very dangerous and also against the law.
Think of it like building a toy car out of LEGO bricks. An improvised firearm is like building a toy gun out of household items like pipes or sticks. It might work a little bit like a real gun, but it's not as safe or effective.
Just like you wouldn't take a toy car on a real road or use an improvised firearm to hurt others, it's important to leave making guns to the people who know what they're doing and who follow the law.