Tags are great things.
Everything has aspects. You've got yours. The ghoul has his. You don't know what the ghoul's aspects are, exactly, but you can make assessments to find out what they are. Those are skill rolls that you make, and if you're high enough, I tell you an aspect. Good stuff.
There are also aspects that you can place on things. Those can be done in two ways: maneuvers and declarations. To get really technical on the FATE system, when you made declarations earlier, what you were doing was creating aspects. Maneuvers are you actually putting an aspect on something. So if you turn out the lights with an action, you're using a maneuver to create an aspect of SUDDEN DARKNESS or something of the like.
Now, to get back to tags. Normally to use an aspect, you have to spend a FATE point. If you know the guy you're fighting has the aspect THINKS GIRLS ARE ICKY, you can use that to your advantage against him. Maybe you want to gain +2 to a Persuasion check or whatever. You spend the FATE, you get +2. It's easy to find an aspect to handle most situations. For example, earlier on your Alertness roll, you could use your Aspect of Undead Slaying Police Officer because police officers have to be attentive. Find something that matches, and use it.
A tag is different. When you first learn of an aspect, the first use is free! No FATE points to spend. So, in effect BlueDevil is making a declaration, creating the aspect of GHOULS ARE TOP HEAVY on the ghoul, in order to give you a free +2 (tagging the aspect) to your roll. You could use the Aspect again later, but it would cost a FATE point.
This allows some creative play that avoids the usual problems of introducing new rules into the game. The first time is free! The second time is costly. Be creative and get free bonuses! But, you have to spend actions to create those aspects to tag, so its all balanced. As a side note, this is why maneuvers are so powerful. If you spend a few round setting up maneuvers you can really give yourself so many tags that you'll accomplish whatever it is you want. Given time, you can do amazing things.