ROHS is Restriction Of Hazardous Substances.
It's European ligslation relating to certain substances used in electronics (eg Lead, Mercury, Hexavalent Chromium. . . etc)
It's rather complicated, but it basically means that certain substances MUST be below a certain level in order to compy with ROHS legislation. If it doesn't comply, we shouldn't be using it. . . And it shouldn't be sold.
The best example is solder. Now ALL solder must be lead free (unless it is used for millitary or medical equipment)
BUT NOTE that it doesn't only cover the use of lead in solder.