This is all part of the Pokemon Puppet Theatre, and the song KanaHana sings as Shellder is titled “Amai Yuwaku”, which means “Sweet temptation”.
Kana Hanazawa’s Shellder expresses its desire for Slowpoke’s tail, which is famous for having a good flavor.
The Shellder also imagines what might happen if it actually bites down on that tail and turns into Slowbro… or Slowking.
While the Shellder is singing about the infatuation, its also sings about the worries about what might happen if that relationship deepens… in other words, the both evolve as a Slowbro.