@famous57 Staple cards are those that are in a high percentage of decks, and consequentially are considered worth owning a playset of (typical 4x copies of a card). For instance, if you've looked at deck lists around here you might have noticed that most decks play a four-count of Ultra Ball. This because that card is universally useful, as it allows you to search out Pokemon from your deck - something almost every deck is going to need.
Staples are frequently :
Items, such as Ultra Ball, Choice Band, Field Blower, or Rescue Stretcher
Supporters, such as Cynthia, Guzma, N, and Sycamore (the latter two are going to be 'rotated out' here in a few months. There have been some decent predictions as to what will replace them by players much more qualified to speak on the issue; consequentially I will keep my mouth shut. Predicting future metagames requires a deep understanding of the current environment - something that I am sure you are working on, but there is no need to worry about all of the complexities for the sake of this post)
And occasionally :
Stadiums, such as Brooklet Hill (for anyone considering playing Water or Fighting decks) and Parallel City (also being lost to rotation)
Pokemon, such as Oranguru (Sun and Moon Base Set - although this is typically a 'one-of'), Tapu Lele GX (which will hopefully go down in price from it's current $30 once the new alternate art version is released), Tapu Koko (the promo SM31 - again, not often a 'four-of', but it sees play in a decent amount of decks that rely on spread damage or that just need an extra 20 damage to hit good numbers), and Vulpix (the Guardians Rising card with the 'Beacon' attack. It's fluctuated in popularity, but it's sometimes a 1-2 of in decks centered around Stage 2 Pokemon).
Arguably the best way to figure out what cards are good to own is to pay attention to what sites like Pokebeach seem to point out as useful, or that are often seen in championship-level decks (even if you aren't planning on rushing out to your next Regionals, it can be worthwhile and a lot of fun to check out
http://limitlesstcg.com/tournaments/ to see what's good and what has been good in the past). Just keep an eye out while you play or read about the game - you'll get a sense of what's good and what isn't in no time.