Pickles are the general term for Pickled Cucumbers or Gherkins.
Its a cucumber that is pickled in brine, vinegar or another solution and then left to ferment for period of time in total immersion in acidic solution or by souring by lacto-fermentation (according to wikipedia)
They are pretty popular around the world and there are people who go mad for these things (not me though!)
Unfortunately, or fortunately (depending on which viewpoint you take) they can’t. Pickles are far too acidic for guinea pigs and will cause them harm if they eat them.
They are definitely a food to steer clear of.