Applying on an ad is a way of contacting ad renter to let him know you are interested by his accommodation. By applying, you push your profile to the renter and give him more information about your application, ideal move-in date and personal situation. Once you have applied for accommodation, you can chat with the renter and ask all your questions.
This is the step before making a reservation: applications you sent are accessible in your inbox. Once you have made your choice, you can make a reservation for one of the properties you applied to from your inbox.
Applying isn’t committing. You can apply on several different ads, and there is nothing to pay at this stage.