A branching random walk is like a game of follow the leader, but with lots of different leaders who go in different directions. Imagine you and a bunch of your friends are playing a game of tag in a big field. You start off in a group, but as you run around, some of you might see something interesting and decide to go in a different direction.
Now imagine that instead of a group of friends, you have a bunch of little particles that are also running around randomly, but each particle can have lots of "children", or particles that follow it around. This is like the game of tag, but with lots of different leaders and followers.
As these particles move around randomly, they can also "branch" or split off, creating more particles that go in different directions. It's like if you and your friends were playing tag, and every time someone tagged somebody else, they would suddenly turn into two people who could go in different directions.
Branching random walks can be used to study how different things move around randomly, like how heat spreads through a material or how diseases spread through a population. By understanding how particles move and branch, we can make predictions about how things will spread and move in the world around us.