What is "Develes Te'enim"?
Rashi: When figs are moist, they are called Te'enim. When they are pressed into a ring, they are called Develes.
Why did he use pressed figs?
Rashi (from Mechilta Beshalach va'Yisa 1): This was a double miracle. Even healthy flesh, when you put Develes 1 on it, it decays. Hashem puts something that wounds into what was wounded, and it heals!
Radak (Melachim II, 20:7 citing Tanchuma Beshalach 18): Similarly, Hashem used bitter wood to sweeten bitter water (Shemos 15:25).
What is the meaning of "va'Yimrechu"?
Rashi: They will smooth it, so it will stick to the boil.
Radak: This is making a bandage or a dressing for a wound.
What is the meaning of "v'Yechi"?
Radak: It will heal. This is like "Ad Chayosam" (Yehoshua 5:8).