Pokémon is a fairly complex game series in a competitive sense. The game is simple to learn and once learned even simpler to beat the plot of the games, but when playing competitively, it can get rough to really be a genuine Pokémon master. It stands to reason that new players are in an even worse position than most vets of the series, generally not knowing the mechanics of Effort Values, Individual Values, etc.
Thankfully, Pokémon players with a social media presence have devised a plan that's sure to be a tradition in years to come. They have decided to publicly showcase a program to utilize the Surprise Trade function on Pokémon Sword and Pokémon Shield for seasoned players to trade away superfluous Pokémon that they've bred or caught to benefit new players, including those who have gotten the game for the holiday season.
This is an excellent opportunity for players new and experienced to trade Pokémon with each other for an additional benefit as well, for if you have enough traded Pokémon in your storage box inventory in the game, the "Roto-Loto", a lottery system based on the number of individual players' Pokémon that you currently own, will be more likely to yield various significant prizes. This includes PP Ups, PP Maxes, Rare Candies, and even the coveted Master Ball.
Why is the Master Ball so important nowadays? Well, when you do Max Raids in Pokémon Sword or Pokémon Shield, there is a slim chance that the Pokémon you will be facing is a Gigantamax-variant Pokémon (meaning it's literally gigantic, with a veritable form change to it). If you beat a Pokémon in a Max Raid, you have a single chance to catch it, and if you fail, it flees (and you'll have to do another Max Raid all over again to find it). Gigantamax Pokémon have an abysmal chance of capture, so the Master Ball, a Pokéball that has 100% chance of capture, is sought after by many players. Only one is guaranteed to players at the end of the plot, though, so getting more is a huge deal if players want to ensure they get the elusive Pokémon in their Max Raids (especially if they're shiny!).
Now, as for what should be traded off, Pokémon that you have bred to get good stats for competitive play are almost always a wise decision (new players don't really mind any stat-line, but it just makes the experience more enjoyable). Extra starters are often a good choice to trade away, as are fossil Pokémon, version-exclusive Pokémon, and Pokémon that are rather difficult to get ahold of in the first place (think Ditto, or other such things, for example). Or, you could express a bit of snark and send naughty players Rolycoly.
If this appeals to your spirit of holiday giving, trade away! Have a good time giving new Pokémon to new players, and enjoy the benefits of interacting with many other players!