If you have significant experience:
"My second marketing position role was with a medical supply company and focused on a print advertisement campaign. It was a team project, and I loved the collaboration with different departments. I immersed myself in understanding the products and how the target audience could benefit from them. After that, I landed a job with a fitness and nutrition startup company that wanted to introduce and define its brand. There was very little existing marketing in place, so I had the opportunity to develop a strategy from the ground up, working closely with the company's founder to set goals and develop effective campaigns. This position was an awesome learning experience, and I refined my cross-platform strategy quite a bit!"

Avoid repeating your resume, and don't assume the interviewer hasn't read it just because they asked this question. Highlight the most significant experiences that are relevant to the skill sets noted in the job ad. If you are new to the field or just out of college, discuss the courses you've taken and internships you've had that will allow you to excel in the role.

If you don't have much on-the-job experience:
"Even though I haven't had the opportunity to work in the field independently, I did complete an internship during my last year of undergraduate studies. I worked with a mortgage company to develop a new LinkedIn strategy and contributed to the kick-off of their print brochures. I researched the real estate industry and the different markets the agency was targeting and gained a lot of experience writing effective calls to action."