A taglock, in simple words, is a personal item that creates a link to that person. For example, hair, bodily fluids (blood, spit), a picture, fingerprints. It can also be as simple as their name on a paper. (I'd like to mention that while bodily fluids/parts are mentioned, I'd stray from using urine/feces. This is a personal belief and quite honestly if you want to, go for it. But I see it as waste, something the body has already expelled and therefore not locked to that person).
If the intended target is a location, instead of a person, you might use a key or maybe some chipped paint, something that would tie it to that specific location.
To use a taglock, you would simply insert it into your spell jar, poppet, bag, or whatever you are using to create the working. Adding the taglock, locks the spell to that one specific person or location, as we mentioned above.
A more detailed example: say you're doing a protection spell for someone. Gather whatever ingredients you prefer using (herbs, crystals, etc), you would add whatever taglock you have to target that person specifically.