Carbon sequestration is when carbon is artificially stored to reduce global warming. The main idea is that if you take CO2, which is a greenhouse gas, out of the atmosphere you can lessen the greenhouse effect and global warming. The methods proposed are taking CO2 and dissolving it into the ocean. The ocean naturally holds a lot of CO2, but recently the levels of dissolved CO2 have been rising, which isn't surprising because there is also more CO2 in the atmosphere. Now having incredibly high CO2 levels in the ocean would also be a bad idea because it causes the ocean to become more acidic. Another, more ironic method is to pump CO2 back into the earth. For example into empty oil holes (which are partially to blame for the CO2 in the 1st place) where it can be sealed off from the surrounding atmosphere. Smarter methods might be reforestation or algal farms that will convert the atmospheric CO2 into O2. I personally like the idea of algal farms because in addition to using CO2, there has been some research into using algae to produce commercially available ethanol. So we could actually convert CO2 into energy you could use to run a car or something.
When it comes to the future, there are three kinds of people: those who let it happen, those who make it happen, and those who wonder what happened.