Plumber's putty is like play dough that grown-up people use to fix pipes. Imagine you have a toy kit where you can build a small city with a toy house and a toy bridge. When you are connecting the parts of the toy city, you need something to hold them together so they don't fall apart. Plumber's putty does that exact thing for the pipes in your house.
When grown-up people are working with pipes, they use plumber's putty to seal the areas where two pipes connect. It's like putting glue on the outside of two cardboard tubes to stick them together. The plumber's putty is squishy and malleable like play dough. It feels like you can mold it into any shape you need to fix the pipes.
When the plumber's putty is molded into the right shape, it goes in between the two pipes that need to be connected. It covers the area where the pipes meet so no water or air can escape. It creates a tight seal so that the pipes stay together and work correctly.
So, in summary, plumber's putty is a type of play dough that grown-up people use to connect and seal pipes in their house.