mandatory cap时什么意思

mandatory cap时什么意思
That is the objective of the many global warming bills now circulating in Washington. The best of these would put a price on carbon through a mandatory cap on emissions from sources like power plants and cars, thus making coal and oil relatively more expensive while driving the market toward cleaner
sources of energy.
我没有理解它在文中的意思,能帮忙解释一下吗?