One of the stats of some Gear Items are the Tag, simply extra effects that it gives. This happears as an red text and have the "tag" word next to it in the item description
Some Items have static tags because it were applied already when it drops, those are statics of that item an cannot be applied anywhere else, also the tag stat number is rng when dropping.
But if your item doesn't have it or you want another tag effect than the one you got from drop, you can use an re-tag item for it.
Basically, you can use re-tag at enhancer to get the tag effect on the item. This costs money and the item itself.
There are different rarities among re-tags, white > green > blue > yellow > red > purple. The higher the better.
Also there are different types of re-tags depending on where it can be used, Weapon, Armor and Special which are made usually for weapons too.
There are 2 type of re-tags when you receive it, the ones with fixed effect that it says literally what effect will add to the item and the random ones, which gives you an random effect of all the list when applied.
To tag an item you don't just need an re-tags to apply the effect, there is an progress bar that you need to fill with another re-tags items that will show the chance of succefull adding the tag.
Higher rarities of re-tags offer more progress to the bar when used to fill it, but also trying to tagging an item with an high rarity re-tags requires way much more re-tags to fill its progress bar.
You can dismantle re-tags to get Zirconia Sand, which can be used to craft high rarity random re-tags.
The best tag effect are: Extra Damage vs bosses, Armor break,.. I recommend you checking internet at your time for proper update info.