Depletion force is a type of force that happens when small particles are mixed together in a solution, kind of like when you mix different things together in a drink.
Imagine if you put marbles in a jar of water. The marbles would sink to the bottom because they are heavier than the water. But, what if you added some smaller beads to the jar? These beads would fill in the spaces between the marbles and create a kind of barrier that would make it harder for the marbles to move around.
Now, let's say you kept adding beads until they filled up most of the empty space in the jar. At this point, the marbles would start to feel a force pushing them together because they are trying to move around in a smaller space than before. This force is called the depletion force.
In simpler terms, when you add lots of small particles to a solution, they get in the way of larger particles and cause them to pull together. This is kind of like when you and your friends try to play a game in a really small room - you end up bumping into each other and getting stuck together!