Ok. This is the last time I'm going to do this.
Let's say you have a picture of you, your mom & dad, your sister (Barbie) and her husband (Ken), and your dog (Chopper).
You also have a picture of just your mom & dad. And another of just you and Chopper.
MC has the capability of storing a list of people that are in your pictures. So if you wanted to find all pictures with you in them, you could just open a view scheme that uses People as a field. Let's say you chose the People\Places\Events view scheme.
Using MC's panes (this doesn't work if you don't use the panes), you can open the People\Places\Events view scheme and scroll through the People pane until you find your name. Click on it, and the list of pictures will show you every picture with you in it. In the example I made up above, that would be 2 pictures. One of your entire family, and one of just you and Choppper.
If you decide to see all pictures with your mom & dad in it, you'd just click your mother's name, then hold control and click your father's name. Now both names are selected and you will see all the pictures with either your mother or father in them. In the example I gave above, this would list 2 pictures. The one of you and your entire family, and the one of just your mom and dad.
So MC provides a really nice way of searching for photos with certain people in them. But how do you get that list of people entered in such a way that picture 1 has 7 people assigned to it, the second only has 2, and the third only has 2?
That's where Tagging Mode comes in. When you opened up the People\Places\Events view scheme, you noticed a list of names under People, a different list under Places, and yet another list under Events. Ignore Place and Events for now.
Under People you would have:
Barbie
Chopper
Dad
Ken
Me
Mom
With this list you can easily select the names of the people you're interested in, and MC will show you all pictures that have those people in them.
Above the three columns (people\places\events) is a button labeled Tagging Mode. Click it.
If you don't have any pictures selected, MC will display some helpful text where the list of People used to be. Forget about that and just click on one of the pictures.
You'll now notice that where the People column is, you have your list again:
Barbie
Chopper
Dad
Ken
Me
Mom
Only now, they have checkboxes next to their name.
If you select the picture of you and Chopper, you'll see that there is a check in the box next to "Me" and "Chopper".
But what's this? You just noticed that Mom is sitting in the background. You'd never noticed that before. So it's time to add her to this list.
Check the box next to "Mom". You'll now have checks next to "Chopper", "Me", and "Mom". You've just updated the list of people that show up in the picture of you and Chopper.
Click the Tagging Mode button again. The checkboxes disappear from the People column. You're left with just a list of names again.
Click on "Mom".
You'll now notice that rather than just the 2 pictures from before, 3 pictures show up.
The same thing works for any field. You can easily change the name of a band to a different band, reassign a song's genre, or year, or album, or whatever. Just by checking a checkbox. It's just that when dealing lists, that Tagging Mode gets really handy.
For instance, to do what I outlined above with the old File Properties, you'd have to select the picture, go to the People field in File Properties and add ";Mom" to the line that reads "Me;Chopper" so that it reads "Me;Chopper;Mom".
For this example, that may not seem like a big deal, but what if you just got a new photo of you, mom, dad, Ken, Barbie, Chopper, and Lassie? Is it easier to check 7 checkboxes? Or is it easier to type "Me;Mom;Dad;Ken;Barbie;Chopper;Lassie" in the People box for that picture?