as DrPumpkinz stated, it's short for "Pay To Win".That is fair, but what are "p2w elements"
Generally referring as "you will be at a disadvantage if you don't invest additional money other than the game purchase", and in this scenario, the p2w element is that if you do not own at least one of Sword&Shield (and the DLC), as well as Legend Arceus, you are at a disadvantage compared to other players, due to not having independant access to the one-of legendaries of both games that are not available in SV (particularly egregious currently due to both Urshifu and the Genies being legal and strong in their own right).
As rule of thumb for wannabe-esports, you never want to have P2W elements. Every player should be on equal terms assuming they own the latest iteration of the game and are willing to commit the dedicated game-time to prepare.
Note that this does not include DLCs, DLCs are part of the "main game" and are equivalent of requiring you to own the latest version of the game. Smash players always assume you own the various DLC releases, League (and Overwatch2 for what matters) players will always assume you own every champion if you remotely plan to play competitively, etc.