Reduction of Hazardous Substances was adopted in 2003, by the European Union. It controls ten substances, which you can read about at Wikipedia. Perhaps the most important substnace is lead, as it has been used in solders since the beginnings of the electronic age.
Let's start with how to pronouce RoHS:
How to pronouse "RoHS", by GreenSoft Technology
We are no experts in this fied, but we pronouce it either "Roze" or "Row-has" here at Microwaves101.