Up=Voltage Protection Level of a surge arrester
There are different kinds of surge protectors. Each with it's own properties.
Some types protect against lightning. Others to supply overvoltage.
The main task of the surge protector is to prevent the surge to propagate ( micro seconds) through the rest of the system. It does in many situations not protect against equipment failure due to the surge.

Example:
The mains side of a power supply can get damaged due to the surge. But the secundaire side remains free from damage. In other words the part where persons could be in danger due to the surge. 

It would be to much in the format to explain everything. 

An excelent explaination can be found in: 

http://www.electrical-installation.org/enwiki/The_Surge_Protection_Device_(SPD)