As you reduce the amount of carbon emissions (the y axis) the methods to keep reducing carbon cost more (the x axis.)

This great graph came to my attention from this video from vlogbrothers. It also has some good explanations of what it means.

Note that carbon capture doesn't really make sense till you've exhausted all the other emission minimizing methods.

Source: https://www.edf.org/revamped-cost-curve-reaching-net-zero-emissions