If Umi has been activated and is on the field, then Electric Jellyfish can negate the activation of a monster effect or spell card, which will then cause it to gain 600 Attack and Defense Points
If a Continuous Spell or Trap copied by this card would have an effect that requires you to activate the card in question (Call of the Haunted, Fiendish Chain, Fire Formation Tenki, etc) then this card doesn't really do anything because it just becomes the effect, and doesn't re-do the activation part of it
Valuable full-art trainer cards can reach into the 100s, with favorites like Lisia, Cynthia, and Misty among the most beloved trainers in the Pokemon TCG
This card then destroys the spell or trap
Your goal is to drop your opponent down to 0 LP, force them to exhaust their deck of cards, or achieve one of the alternative win conditions like the famous Exodia strategy