The phrase "at its peak" is very similar in meaning to "in its prime". But the "peak" of something is the very highest level of performance, while the "prime" is a longer period of generally good performance. An athlete's "prime" might last several years, but their "peak" might only be a single season.

The "peak" of a mountain is the very top. It's the highest point on the mountain. So you can imagine that, if a person's skill or strength was recorded on a graph, the highest point of the graph will look like the peak of a mountain. This is why we call this point a "peak".

Some other phrases that include "peak" used in this way include:

• To "reach a peak" means to gain the highest level that you've ever achieved at something.
• "Peak performance" is the highest level a person is able to perform at.
• "Peak production" is the fastest speed that a factory, industry, farm, etc. can make something.