Ancient Trait Pokemon are pokemon with a so called ability this can vary to be defensive or offensive so consider having one or two of these on your team. The way you can identify if they are ancient trait is because of a symbol with a description at the top of the card underneath the name of the pokemon so take this image of a macargo as an example: You see the name, look underneath the name and there is the ancient trait (This also applies to secret rare, EX, Dual Types & Most likily GX)
An ancient: This is sceptile