If you're at the forefront, you're either the very first to do something or you're in the most important position. Inventing an effective new vaccine would put a scientist at the forefront of modern medicine.
The word forefront has been used since the 1600s to mean "front part." Originally, it referred to architecture — you might have commented on the way windows were placed on the forefront of a building, for example. Later, it was used to describe the soldiers at the very front of an advancing army. Today when we talk about the forefront, we usually mean the highest ranking place or position: "She's an amazing comic book artist who's at the forefront of contemporary graphic art."