A well-defined group of consumers (customer segment) whom your business wishes to cater to with your product or service offerings. When you have a well-defined group of consumers, it becomes easy for the business to make marketing decisions as the focus segment is clear. You align your marketing ideas to fulfill the requirements of the selected consumer segment and hence leading to better marketing campaigns.
Here is an article which can provide you with more insights about target marketing strategy: https://www.feedough.com/target-market-definition-examples-strategies-analysis/