The Fishing Pole of Love is a special type of Bamboo Fishing Pole which, when the suction cup at the end of the line attaches to a person's chest, leaves a red mark. This mark then slowly develops into a full formed Carp-shaped mark and as the mark grows, so too does the person's feelings of love towards the person who used the Fishing Pole.
Whilst out searching for gifts for Akane, Ryoga came across a basket filled with Fishing Poles of Love. The Salesman explained to Ryoga how the Fishing Pole would be able to cause whoever he used it on to fall in love with him. Despite with being initially skeptical of the Salesman's claim, Ryoga buys one of the Fishing Poles.
Once Ryoga returns to Furinkan, he tries to use the Fishing Pole on Akane, but unfortunately ends up hooking Ranma instead. Sure enough, the magic of the Rod takes effect and Ranma begins having romantic feelings towards Ryoga, which cause him to start acting like a wife towards Ryoga. (Ranma would also turn into his female form in order to make Ryoga "more comfortable" with the situation.)
Ryoga soon tires of Ranma constantly being around him and decides he needs to kill Ranma so that he and Akane can finally be together (as Ryoga believes Akane now knows his feelings towards her). However, as Ryoga attempts to kill Ranma, Akane (who thinks that Ryoga's become so lonely that he's chosen Ranma instead of a "normal girlfriend") arrives and tries to explain to him that this is all because of the Fishing Rod. This is the first time Ranma has heard about the Fishing Rod and he almost instantly takes it in order to use it on Ryoga so that he'll love him back.
In the ensuing fight, Ryoga uses his bandana in order to reflect the suction cup that acts as the "hook" of the rod onto Ranma's chest before punching Ranma into the air, which rips off the Carp mark from Ranma's chest. It's unknown what Ryoga did with Fishing Rod after this, but it is easy to presume that Ryoga disposed of the item.