Profile characteristics are data about, for example, your demography. Think of age, gender, education and so on. These are often asked to be completed by online survey sites & consumer panels where you as a member can earn money by completing surveys .
Based on these characteristics, the survey site can determine whether a survey is interesting for you or not. This is because the questionnaires must always be conducted among a specific target group. This is determined by the client.
Then it may happen (more often) that you are invited to a survey where you do not appear to fall into the target group. Then you are often informed after a few control questions that you cannot participate. A waste of your time and you don't deserve anything. That is why we recommend that you always fill in the profile characteristics.