Water resists Steel: Steel equipment cannot damage water. If you drive a chainsaw into a pond or try to fire a bullet into the ocean, you'll see what I mean.
Bug resists Fighting: Fighting refers to martial arts mostly. Bugs are mostly small and karate-chopping them is kinda hard. From a more logical perspective, bugs have hard exoskeletons (unlike most vertebrates, which have little to no exoskeleton and stronger endoskeleton). If they are as big as say Heracross, then physically harming them from the outside would be pretty hard.
Fighting resists Dark: Fighting here is martial, or uniform arts while Dark (Evil type in Japan) refers to dirty, or untrained, play. Martial artists train their bodies and attacks to be able to counter those who aren't trained, and are hence more durable and powerful against them.
Steel resists Fairy: In the olden/fairy tale days, there were loads of superstitions about iron being anathema to mythical creatures like fairies. To put it simply, metal is strongly disliked by fairies to a point it doubles up as a weakness.
Poison resists Fairy: Fairies, in most cases, are displayed as nature spirits. Since poison contaminates the environment, fairies will want to be as far away from poison as possible. Again, like with steel, this strong dislike doubles up as a weakness against it.
Electric resists Steel: Again, steel equipment fazes right through electric currents. When steel comes in contact with the current, the current flows through it and the equipment conducts it without complaint (mostly). No harm is done to either, but a current strong enough may burn through a conductor.
Hope I helped!