If you are well equipped with CTR, it will come only as a surprise if you don't have an idea about Conversion Optimisation since both of them are correlated.  A conversion is made when a user clicks on something the businesses want them to click on, i.e., to buy their products or services and a higher conversion rate means a successful business campaign for the brand. Regarding conversion optimization, the interviewer wants to know your technical marketing strategies to optimize this conversion rate for its brand since this is the main core concern or target for all businesses. To achieve a good conversion rate, you should talk about various marketing strategies you have in your mind that will result in making your brand visible to more and more people and constantly expanding its reach to a newer audience.