Prim tags are a system that has been newly introduced with the v4 series. Its a fairly simple concept, which allows for a lot of interesting tricks and possibilities, which will be explained in detail here.
First of all, lets explain what a prim tag actually is. Basically, a tag is something every prim may have. By default, most prims in the v4 series have no tag, meaning they will act like normal prims. However some do posess a tag by default, which makes them show up in special conditions. The most basic idea of a tag is, that if a prim has a tag, it will only show up if said tag is used in the state change command for the penis.
To explain this a bit better: Lets assume you have a prim for a set of barbs at the tip, and you would like these barbs to be able to either show or hide, no matter which state you are in. If you have 8 different states for your bits, you could either create a duplicate set, one with, one without states, or you could assign a tag, for example "barbs" to the prim for the barbs. Now, if you say "/5 hard", you will get the normal, erect shaft. If you say "/5 hard.barbs" the barbs at the tip will show up. So if you want to use a tag along with a state, simply append it with a dot in between.
This has other uses, for example if you have a knot on your penis which you would like to show or hide depending on what you input, you can use "/5 state.knot", or even chain tags, like "/5 state.knot.barbs". The system already does this internally for certain operations.
Another use of states is to create cum-cover or condom layer prims by tagging them appropriately. This procedure is explained in another tutorial which focuses on that particular thing.
So how can you actually assign tags? The procedure is easy. Once you have linked and set up the prim(s) you want to tag, simply use "/5 addtag <prim name> <tag name>" and youre done. If you wish to remove a tag, use "/5 deletetag <prim name>". As a reminder: <prim name> stands for only the part behind the "!penis" part.
What else can tags do for you? Quite a lot if youre creative. As mentioned in other tutorials, they can control the cum-layer prims, and can add optional displays of jewelery or other features to your bits, as well as even create clothing that can react to the different states that your bits have.