To begin with, recall that ethics is the branch of philosophy that analyzes and defends concepts of value and thereby seeks to determine right and wrong. Ethics can evaluate actions in lots of different ways. For instance, it can evaluate the consequences of an action, or its intention instead. But virtue-based ethics evaluates an action in terms of its relation to the kind of person someone is.
There are lots of ways that the kind of person you are affects, or is affected by, your actions.
So actions can express or reinforce your character, but they can also develop it.
Virtue-based ethics tends to split up character traits into those that are considered morally desirable, and those that are not. The former are called virtues, and the latter are called vices. Here are samples of both.
Philosophers that advance virtue-based ethics usually claim that everyone should try to cultivate virtues and avoid vices. This tells you what type of ethical theory this is.
Ethical theories can evaluate an agent’s actions and the character of the agent. Types of theories, such as utilitarianism and deontology, make actions the primary object of evaluation. Here, the character of the person is only seen in light of their actions. Therefore, character is secondary for these theories.
By contrast, virtue-based ethics makes character the primary object of evaluation. Here, actions are understood in terms of how they impact the character of the agent.
You would also evaluate actions in terms of how they encourage certain virtues or discourage certain vices.
If a naturally shy person made an effort to take up public speaking any opportunity they got, you would see this as a good action because it’s an attempt to overcome the vice of excessive shyness or bashfulness.
But if someone was already very confident, then speaking publicly all the time might be seen as a sign of arrogance. In this case, this action is bad because it encourages the vice of arrogance.
Because of this emphasis on character, the kind of question you would ask yourself is different than the kind that you would ask yourself if you were primarily concerned with evaluating actions.
A character trait of moral disvalue
A character trait of moral value
A theory of ethics that maintains that an action is to be evaluated based on how that action informs the aspects of the agent's character