A character sketch is when you write or talk about a person and describe what kind of person they are. Imagine you are drawing a picture of a person, except instead of just drawing what they look like, you are also describing what their personality is like!
So instead of just saying someone has brown hair and blue eyes, you might say that they are very kind and always try to help others. If someone is grumpy and doesn't like to talk to people, you might describe them as being shy or introverted.
Writing a character sketch helps you understand the person better and to see them as a whole person, not just a list of traits. It's like painting a picture with words, and it helps you really get to know the person you're talking about.