Hey, dacosta, welcome to the forum. As jpgilham said, you also get reimbursed. So, yes, you do go to a restaurant and spend $25. While you eat (and drink) you make the observations required in the guidelines for the shop. You keep your itemized receipt, documenting how much money you spent. When you get home, you complete an online report and you upload a photo of the receipt. When you are paid, usually 30-60 days later, depending on the MSC, you get reimbursed for the food costs, up to $25, if that was the agreement under which you accepted the assignment, along with the fee, of say, $5.
Definitely, earning $5 is not very much. Whether this type of assignment is worth it or not is a call you have to make for yourself. Some mystery shoppers do not like this type of shop and do not perform them. I personally LOVE them. I have a very rewarding full time job that I love (but the pay sucks!) and I mystery shop for "extras," especially eating out because I LOVE to eat in a nice restaurant. When I do these shops, I do them only if I want to eat in that particular restaurant because I enjoy the restaurant and the food..... and only if the reimbursement is enough to cover two or three so I can take a family member. And I consider that I am "paying" for the dining experience by writing a report. I'd rather write a report than cook.