The star is an indicator of how rare the card is.
In the Pokemon Trading Card Game, most Pokemon cards fall under one of three main rarity categories: Common, Uncommon, and Rare. The rarity of the card is just an indicator of the probability of obtaining any card from the Booster Packs of each expansion. The rarity of a Pokemon card is shown by one of four symbols: a black circle for Common cards, a black diamond for Uncommon cards, a Black star for Rare cards, and a white star for Ultra Rare and Secret Rare (subcategories of Rare) cards.
So, which cards are Ultra Rare and Secret Rare? Well, according to Bulbapedia:
[Ultra Rare cards] are marked with [a white star] and are typically associated with a unique card classification or mechanic such as Pokémon V
and:
Secret Rare, which are cards with set numbers outside the printed size of the set (for example, a card numbered 101/100), marked with [a white star], and have a unique artwork scheme such as a Rainbow Holofoil print.
So basically, Ultra Rare cards are the more powerful Pokemon cards, such as most Pokemon EX, Pokemon GX, and Pokemon V cards. Secret Rare, however, is an even rarer type of Rare card that features the same cards as Ultra Rare, but with special artwork, like a rainbow holofoil across the card.
Based on your description, I believe your card is this card (click the link to see it):
https://ibb.co/K6LPCxc
If it is, then your card is Ultra Rare, and especially rare because of the golden layout in the background.
Source
Anyway, that's what the star means! Hope this answers your question!
