PhotoMesa supports several ways to "annotate" your photos - this is just a fancy way of saying that you can add extra information to the photo that is about the photo. You might want to add a caption describing something about what is going on in the photos - or you might want to label who is in the photo (even recording where individuals are located within the photo) - or you might want to describe other objects that you may want to look for later (such as animals, specific types of art, instruments, or anything else that you photograph frequently).
In PhotoMesa, you can only annotate the photos that are already visible. So, first show a set of photos that you want to annotate (by selecting their folders, searching for photos, etc.). Then, click on the Annotation tab:
There are two kinds of annotations - People and Categories
You can annotate photos with the list of people that are in them. Do this by creating new entries in the People section of the Annotation pane. Enter someone's Given and Family name into the boxes and press the "Create" button. Then, there are several ways to annotate a photo with that person's name. Some ways are designed to be efficient, and let you annotate several photos with a single name - this "bulk annotation" is useful when you have taken several shots with the same person or group of people.
In addition to people, you can create your own hierarchical structure of object types to annotate your photos with. Do this with the "categories" portion of the Annotation tab using either of the first two mechanisms described above for people annotations.
You are free to choose any classification categories you like. You might choose to create a top-level category of "Location" in order to track where you take your pictures - or you could choose "Photo Type" and create sub-categories of "Portrait", "Landscape", "Group Photo", etc.
The point of these categories is that they are entirely custom and up to you. They are your photos, and you are free to annotate them any way you choose.