Okay kiddo, sometimes people don't agree with things that the government or other powerful people are doing. When they want to show that they are unhappy or want things to change, they might gather together with other people who feel the same way and go out onto the street to let their voices be heard. This is called protesting or demonstrating.
Street protesters might hold up signs or yell chants to show what they believe in. Sometimes they march or block traffic to get attention. The protesters hope that by making their feelings known, they can convince others to join in their cause, and hopefully make a change happen.
Protesting can be a way to speak up against things that are unfair or unjust, but it's important to do it safely and peacefully. Police and other authorities might be there to make sure everyone stays safe and doesn't cause any harm. So that's why you might see people marching on the street with signs or chanting, they are street protesters and they are trying to make a difference.