thats for adding on a diff. plug for diff countries OR to add the extension cord for longer reaching.
split cable at where in enters the charging block? Hmm, yes, youd need a new charger.
been winding it too tight?
Contact Apple for a look and replacement, or you can purchase a replacement. But see what your Apple store says.
Careful with your charger and its cable
Do not make any hard bends or folds in your charging cable, or wind it tightly, always make either circles or loose loops when winding your cord up for storage. Also do not, as many people have seen, unroll your charger block from the magsafe end by letting the charger drop and unroll itself like a yo-yo, this is both hard on the charger and its connection points at both ends.