Roman numeral code: V / V
Scale notes: 2, #4, 6
Attributes: boldness, power, surprise, unexpected
5 of 5, also called the secondary dominant, adds power and excitement to music. It is frequently heard in all types of music.
5/5 is a chord borrowed from a different key: the key defined by the 5 chord. It is the 5 harmony of that key. So 5/5 bears the same relationship to 5 that 5 bears to 1.
Notice how it sounds like it is outside the confines of the original key, and serves to accent the 5 harmony.
The sequence 5/5 -> 5 -> 1 is an especially powerful cadence.