If you consider the path that a user takes through your website from landing page to successful conversion, you can think of the number of users that make it to each point along the way to a successful conversion as similarly shaped to that of a funnel. In a typical setup, you may have a very small percentage of your users make it to a successful conversion, but there are several areas along the way that either improve the chances the user will convert or decrease those chances.
As an example, lets say your landing page had 1,000,000 unique users reach it, and 10%, or 100,000 clicked on the call to action button, taking them to the next step. On step 2, lets say that 5% of the 100,000 were able to make it through, which would be 5,000 users. Step 3 is the conversion step, and lets say that only 20% of the remaining users make it through, for a total of 1,000 successful conversions.
If you were to look at it based on raw conversion percentages, you would think that the best step to work on increasing conversion rates would be step 2 as it has the lowest raw percentage conversion rate. However, if you were to make step 2 the first improvement location, you could be optimizing the conversion process for the wrong user. Instead, if you were to optimize step 1 until it reached some conversion rate goal, you could see a much greater number of users make it to step 2 in the process. At that point, you may find out that there was nothing wrong with step 2 itself, as the problem was step 1 all along.
If you do determine that step 2 does not meet its conversion rate goals, focus on optimizing it until you reach the goals you have set. Finally optimize the last step in the conversion process to remove any final roadblocks to conversion for your users.
The important thing to remember is that if you optimize from the narrow part of the funnel outwards, you may be optimizing conversion success for a very narrow subset of users, and when there is a larger stream of users making it further in the conversion process, your optimized step 3 may not work as you had hoped for this expanded audience to your conversion endpoint.