Oliver Twist is a novel written by Charles Dickens. In order to help you answer this question, it is first of all important to point out that Oliver was an orphan who grew up in a workhouse. A workhouse was a place in Victorian Britain where the poor were offered accommodation in return for labor. As an orphan, without any family to look after him, Oliver had to live in a workhouse in order to survive.
However, Oliver wasn't able to continue living in the workhouse forever: instead, he ended up being sent away. The reason for this is the fact that Oliver had dared to ask for more food. In order to understand the significance of this, it is important to understand that life and conditions in Oliver's workhouse were horrible and inhumane. People were starving. Oliver and the other kids were constantly hungry, which is why the children decided to nominate one of their group to ask for more food.
They knew that this wouldn't be a good idea, but they were so desperate in their hunger that they simply had to take the risk. Oliver ended up being chosen to do this, so he asked the master whether they could have some more food: "Please Sir. I want some more." This is the reason why he ended up being sent away from the workhouse.