bananasoing, it's called oxides =) If the oxides are causing the solder joint to be difficult to make you're not using flux. Flux is a cleaner and protector for the joint under formation that prevents it from being exposed to air so that oxides don't form and ruin it, it also decays rapidly under heat. Most solders have a resin core which acts as flux the first time round, subsequent heatings require flux paste to be applied or the joint quickly deteroriates with exposed to the air heating, oxidizing it internally and make it brittle.
The exact same thing happens during stick welding, a sacrificial flux resin is on the outside of the stick of metal to be welded preventing it from coming into contact with the air.