Refers to the very front of a battlefield or, by extension, the leading edge of any field or activity. Often used in phrases like 第一線で活躍する (to be active on the front lines).
He has worked on the front lines of medical care for many years.
その研究者はAI研究の第一線に立っている。
That researcher is at the forefront of AI research.
He is still active at the forefront of medicine.
She retired from the forefront of her career at age fifty.
He always stands at the forefront of research.
The soldiers kept defending the front line.