The glowing yellow aura certain wild Pokémon display in Pokémon Sword and Shield denotes a 'Brilliant' Pokémon. No, we don't just mean a Pokémon that's really ace like Charizard or Grookey - we mean a Pokémon that is of a higher level than the average specimens found in the surrounding area.
Brilliant Pokémon also know 'unusual' moves (such as Egg Moves - moves which usually only appear in Pokémon you've bred) and a minimum of two out of their three stats will have the highest possible value.
In short, they're the 'best' versions of a specific Pokémon species you'll encounter in the wild.