This invitation is one of the highest accolades a mathematician can receive, and is the second such invitation extended to Professor Wooley (the first being for the 2002 Congress in Beijing). The 2014 invitation reflects his breakthroughs on estimating exponential sums; a number of papers relating to this are available via his staff webpage.
The ICM, hosted by the International Mathematical Union, is the largest conference for the topic of mathematics. It meets once every four years.