It replaces every character in that character class (basically: the unusual ASCII control characters, plus anything outside ASCII) with:

1. opening square bracket, followed by
2. capital letter "U", followed by
3. plus sign, followed by
4. the numeric value of character being replaced, in uppercase hexadecimal, padded to a minimum of four digits, followed by
5. closing square bracket

It's a regular expression I use quite... regularly. It makes any non-ASCII characters stick out like a sore thumb, so you can see exactly what characters are in your string.