Jewelweed is a plant/flower the blossoms annually. It's flowers are yellow or orange with red spots (called Spotted Jewelweed) and have a distinct bell or trumpet shape (though the mouth of the flower is covered by the petals) that hang towards the ground. They have oval shaped (not perfect ovals obviously) leaves. A neat thing about Jewelweed is that a certain time of year when you touch the flower the seed in the center of it will shoot out. Pretty neat.
Jewelweed, more specifically Spotted Jewelweed, is a topical remedy for Poison Ivy. To use it, its best used right after exposure but still useful a short while later, just cut into the stem of the plant, closer to the ground the beter, and rub the insides on the exposed area. People also use it for insect bites and other skin irratation caused by plants.
The active ingredient is called lawsome. The way it works is that lawsome binds to the same molecules that Urushiol oil, the active part of poison ivy, does and once bound the Urushiol has nowhere to bind and cause the allergy. This is the reason it's best to apply Jewelweed as soon as possible.