A target group is a predefined group of people that is selected on the basis of certain criteria. Consider demographic data. Everything with the aim of being able to conduct more targeted research into a particular issue, or to obtain a more representative target group (for example 50/50 men and women).
If you are a member of a so-called survey site you can earn money there by completing surveys. However, such paid surveys are only of value to a client if the surveys are conducted in a specific target group. For example: if Heineken wants to know what its customers think of a new flavor of beer, they only want to hear from people who actually drink this brand.
Then you cannot participate in a paid survey. Normally you will not receive an invitation at all because you are not selected by such a survey based on your profile characteristics. However, if you do not enter these or do not keep them up to date, this can go wrong.