The Force is an energy field created by all living things. It surrounds and penetrates an individual. You have to have a connection to it to use forceForce powers. The Ysalamiri
The Ysalamiri create a bubble around them which pushes back the forceForce, removing the ability to connect to the Force, as a survival mechanism to avoid predators (VornskrsVornskrs) on their home planet (MyrkrMyrkr). This is now EUThis is now EU as it is from the ThrawnThrawn trilogy which also predates midi chloriansmidichlorians in the prequels. The net effect is that a Jedi (or Sith) cannot use the Force when within a bubble
The net effect is that a Jedi (or Sith) cannot use the Force when within a bubble, and powers like Force lightning invoked from outside a bubble cannot harm a target within one. Another interesting effect is that a clone can be grown at an accelerated rate in a Spaarti cloning cylinderSpaarti cloning cylinder with a Ysalamiri blocking the Force asso the clone doesn't suffer from clone madness. This allows clones to be grown in 2-3 weeks vs. the year or longer normallynormally needed to avoid clone madness.
https://starwars.fandom.com/wiki/Ysalamir
https://starwars.fandom.com/wiki/Spaarti_cloning_cylinder
If you read the Thrawn trilogy, a lot of this is covered in more detail.